[BUGFIX] Convert slashes in Windows paths for blacklist generation

The composer ClassMapGenerator expects the blacklist regular expression
to contain forward slashes. As the realpath of a given file is used it
has to be converted before using it in the expression.

Resolves: #72782
Releases: master, 7.6
Change-Id: Ia61a490be4cee4c8b7c492b4514bff2d720343ae
Reviewed-on: https://review.typo3.org/46039


Reviewed-by: Helmut Hummel <helmut.hummel@typo3.org>
Reviewed-by: Morton Jonuschat <m.jonuschat@mojocode.de>
Tested-by: Morton Jonuschat <m.jonuschat@mojocode.de>
Reviewed-by: Xavier Perseguers <xavier@typo3.org>
Reviewed-by: Stefan Neufeind <typo3.neufeind@speedpartner.de>
Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: Helmut Hummel <helmut.hummel@typo3.org>