[BUGFIX] Autoloader fails on Windows

str_replace fails because of different directory-separator
Windows uses.

Change-Id: Idd152c0cc88fe2d8f726235f411f54617820b061
Resolves: #56619
Reviewed-on: https://review.typo3.org/28648
Reviewed-by: Philipp Gampe
Reviewed-by: Nicole Cordes
Tested-by: Nicole Cordes
Reviewed-by: Nico de Haen
Tested-by: Nico de Haen