[BUGFIX] Respect interfaces as well when checking class existence

As a backwards compatibility measure, the composer class loader
checked for class existence before requiring a class file, which
could only happen if wrong class name casing was used before.

We now enable this also for interfaces.

Releases: master, 6.2
Resolves: #65434
Change-Id: I6dc1dc6cbf8cb7018ceeff1eb881c80658c11e33
Reviewed-on: http://review.typo3.org/37412


Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>