Use classmap to load core files

[rullzer]

We can't use an authoritive classmap in the current state as it would
kill app loading. However we can use a proper classmap for the normal
core stuff.

Performance: https://blackfire.io/profiles/compare/69212b7f-3227-421d-8aff-9b87bd166bba/graph

It can probabaly be even better on a production system. Since that I/O time is just bogus

CC: @LukasReschke @nickvergessen @MorrisJobke @icewind1991

[mention-bot]

@rullzer, thanks for your PR! By analyzing the annotation information on this pull request, we identified @LukasReschke to be a potential reviewer

[nickvergessen]

When do we need to update this? only on release, or also while adding a new class?

[rullzer]

When adding new files.
Note that since we did not make it authoriative it still uses the normal PSR-4 loading when loading classes not in the classmap. So worst case it is a little slower.

[icewind1991]

👍

[MorrisJobke]

Tested and still works 👍

[rullzer]

Drone finished succesfully (see https://drone.weasel.rocks/nextcloud/server/470) but for some reason didn't update github.

Merging.