[BUGFIX] Class loader loads aliased classes twice

Check if a class already exists before requiring the file.

Change-Id: I555d259f7fb869428d20b63d2766684bb21c1a5c
Fixes: #55904
Releases: 6.2
Reviewed-on: https://review.typo3.org/27570
Reviewed-by: Markus Klein
Tested-by: Markus Klein
Reviewed-by: Georg Ringer
Tested-by: Georg Ringer