[BUGFIX] Install tool extension scanner .rst file core version

The core version indicator in the extension scanner of the install
tool on reST files is broken on windows OS since path is split by
/ instead of \. Use DIRECTORY_SEPARATOR to fix this.

Change-Id: I57928468c1a4cae2a439bd55b00874d13f3569dc
Resolves: #82830
Releases: master
Reviewed-on: https://review.typo3.org/54450


Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Susanne Moog <susanne.moog@typo3.org>
Tested-by: Susanne Moog <susanne.moog@typo3.org>
Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: Benni Mack <benni@typo3.org>