The main trick is to not have the players feel like they have to obsessively search every part of every room.
Over time, this is dictated by your actions as the GM. (Obviously there's some switchover time if you're shifting styles.) If you hide a critical clue or tasty treasure requiring a DC 20 Perception check under the bunk of barracks room #57/100, or put a deadly trap on door #23/50, they're either going to have to search everything or just throw up their hands and say "screw it, we'll bull forward and maybe we're missing something."
You don't mention a game system, though you should because different games approach this problem differently.
GUMSHOE would say "Don't require search checks to find important things. PCs find them automatically. Require checks after that to interpret what was found or whatever, but if your game would be blocked by a roll, don't make the roll in the first place." This obviates the problem.
A D&D 3e answer might be "pre-roll a bunch of player perception checks ahead of time, don't make them say it and roll it but just apply those checks whenever they go into a new area, if their stated SOP is to search around everywhere." This doesn't remove the problem, but it speeds it up significantly.
Similar is just when they say "we search barracks #57" - if they have the time to take 20, you don't have to roll, just spend some in-game time (not table time) and tell them what they find (take 10 and take 20 reduce resolution time). Or you can just cut to the chase and come right out and say "there's nothing new here."
A drama-game (or even D&D 4e I think) type answer would be to say out loud to them "Hey guys, important stuff will never be in some generic room, I'm only going to put important information into clearly 'named' rooms, just like only 'named' NPCs are important." This removes the issue at a metagaming level, though at great cost to simulation.
4e and 5e have "passive perception" in an attempt to speed up the game, but your issue is that they want to actively search and not just take the passive chance, so that doesn't help much.
Talk to the players.
The party isn't doing anything wrong per se. Proper use of choke-points is in fact good tactics, especially if they don't have a strong need to move in and surround the enemy. (If the party had a bunch of melee guys getting screwed over by the tactic, then it would stink.)
But the easiest thing might be to talk to the players. Acknowledge that they've done nothing actually wrong but bring up the points you have brought up here, that it's just not very much fun for you, and maybe for them.
Refuse to engage.
To quote the usual message: "If the party can do it, so can your monsters."
The players like to huddle up in a corridor and refuse to come out? Why would the monsters stand there in the open and try to break through instead of retreating and waiting for the PCs to move?
Encounters as laid out in the book are only the general suggestion of how something works, but unless your monsters are mindless or of merely animal intellect, they should be able to easily recognize the strategy and move to the sides, out of the line of fire of the "nukers" in back. Stalemates aren't fun, especially if there's also some incorporeals or patrolling monster squads hassling the back line (though that depends on the specific fight at hand). Keep in mind that fights are loud, and big evocations doubly so; you have every justification you need to bring random wandering patrols down on the party's back. Just be careful that this strategy makes sense in context; if the party gets jumped by patrols when they fight in hallways but doesn't when they go into the room, it's gonna feel unfair (because it is) and set up a players-versus-DM mentality that you really don't want to encourage in a game.
But, back to the point, the monsters could definitely retreat under fire and pull back into their own hallway position, or even leave the area entirely. Too often I see PCs playing with smart tactics while the monsters just growl and run at the party over and over. Only mindless creatures would do that; an obviously overmatched party of intelligent monsters should retreat, regroup, reinforce.
Engage on their own terms.
On the same basic level as above: Anything they can do, you can do. ("Better" is a matter of debate.)
There are very few monsters who totally lack ranged options. If the party likes to huddle in the back and lob spells, arrows, and cantrips, the monsters can absolutely respond in kind. Orcs can throw spears and axes just as well as they can charge in swinging. If the party insists on standing in a tightly clustered hallway position, use their tactic against them. Fireball and lightning bolt can be highly effective against a group that insists on standing in a straight line.
An extended artillery battle might not be much fun, but if they're gonna play games with doorways, you can play right back, and suddenly moving in close to prevent such a standoff seems a lot better.
By the way, keep in mind that you can take an action at any point during a move, so it's completely valid to have a lich standing to one side of the door, out of easy line-of-sight, then run into view, fire a spell off, and move back into cover on the other side of the doorway. They can ready attacks to hit him when he appears, but it's not a foolproof plan since there's also spells like invisibility and mirror image out there.
Enforce the rules
It's entirely likely that any direct attack made while trying to "shoot over your party's shoulders" in a cramped hallway would invoke a cover penalty on the guys in back. Based on the Dungeon Master's Guide rules about cover when using miniatures (DMG p.251, with diagrams on 250), it's hard to imagine any arrangement of characters as described that wouldn't have 3/4 cover against virtually any target they want to shoot at. They might be able to arrange to have merely half cover against some targets, depending on how you read the cover rules, but in general, firing between allies would be a lot like shooting through an arrow-slit, so 3/4 cover sounds right.
However, ranged attacks from the monsters would have the same penalty against the back-liners, so this isn't actually a solution as much as a way to make the party have less fun and even more dependent on area effects (where cover is measured from the origin point of the blast). Everyone getting a +5 to AC will make fights stretch on and on, and makes it feel bad to the players to keep rolling misses.
Spread out the monsters.
Both tactically and strategically. If the party is relying on throwing area effects, make sure you keep your monsters spread out so only a few can be affected at a time, and maybe break up a big fight into a few smaller fights where they'll have to blow through more spell slots if they want to keep doing their shtick. And along that line...
Don't let the party rest easy.
This strategy seems to me to be very spell-slot-intensive. They have to keep the front line healed, and get almost all their damage from big damage spells like fireball. That suggests to me that the party may be sleeping more than normal, and that you might be allowing them to go take a long rest any time they want to.
Pressure the party to hurry up with time sensitive missions. Launch ambushes if they sleep in the dungeon. If they leave the dungeon to rest, the monsters use that time to reinforce their numbers, reinhabit rooms previously cleared, set guards or traps, block doors or build barricades, summon demonic defenders, and so on. Make the monsters at least as tactically smart as the PCs are. Let the party know that relying almost entirely on the spellcasters' damage output is not going to cut it.
The module is only a suggestion.
The module should not be treated as a bible. Encounters can and should move around, DMs are encouraged to add or remove creatures to make the fights harder or easier, and so on. Tune your adventure to the party, if necessary. Don't feel constrained by the words on the page.
If they're depending heavily on spells for damage, counter magic with magic when you can, especially if they keep leaving the dungeon to "wait until tomorrow". The local spellcasters can and should adapt to their strategies by preparing protection from elements, wall spells, even globe of invulnerability if the levels are high enough. Keep in mind that a simple fog cloud can be utterly devastating to any ranged combat tactics, and a wall of fire blocks line of sight in addition to pulsing fire damage onto the party and forcing them to either back off or run through. Old standbys like grease and web are some additional fantastic low level options for making an area very undesirable to the party; personally I love web because of the irony of punishing the players for failing to move around by making it impossible to move around.
Best Answer
Having an invisible Imp floating through a dungeon is quite an investment for a Warlock character, and have the obvious effect of allowing what you describe. There are other "overpowered", scenario-breaking usages, like following a NPC all day long to track the rogue network he's part of, for example.
However, those abilities are well defined in the rules, and in no way the Warlock PC is abusing them while sending his Imp to scout before the party enters.
If you're not comfortable with the situation, you should discuss it with the Warlock player to find how important those features are for him, and if he/she accepts altering his character because you are not comfortable with the situation, despite the PC 100% following the rules.
Another, perhaps better outcome would be to have your playstyle adapt to the situation raised by the invisible-imp Warlock. Like Zeiss Ikon suggests, plenty of things can happen while a Warlock is senselessly waiting for his familiar to map a dungeon. And like Dale M writes, you can probably save time for the whole party while handing a special map to the player, created beforehand because you know he will use this trick.
Now, there are plenty of other things that may happen, or limit the ability to map the dungeon.
That's just a bunch of ideas to help you deal with the player resources. There are creative ways of using one's abilities, but a DM can also use creative ways of keeping the game interesting.