[BUGFIX] Improve resolving path to autoload.php

The patch improves the resolving of autoload.php for
frontend requests in order to support Windows systems
with symlinked core, but copied index.php

Resolves: #70700
Related: #69011
Releases: master
Change-Id: Icede0bd4907ae8ff418df39715b94fc8324a2ad8
Reviewed-on: https://review.typo3.org/44060


Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Markus Klein <markus.klein@typo3.org>
Tested-by: Markus Klein <markus.klein@typo3.org>
Reviewed-by: Helmut Hummel <helmut.hummel@typo3.org>
Tested-by: Helmut Hummel <helmut.hummel@typo3.org>