[BUGFIX] ClassLoader also needs to check for interfaces

An interface might already be defined, check for existence before trying
to require it, which will cause PHP errors if the interface comes from a
different file path.

Resolves: #62212
Releases: master, 6.2
Change-Id: I9cf43402bbcbbb5e27dbebb4ea48a2e1b483f360
Reviewed-on: http://review.typo3.org/33489
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>