[TASK] Use absolute paths when installing extensions

In order to prepare for further refactorings and streamlining
extension manager code, resolving extensions' path is now
always handled with a full path.

Releases: master
Resolves: #93054
Change-Id: I2d853c3f5d4a6ae4861dcd174d380a855764d995
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/64873


Tested-by: Benni Mack <benni@typo3.org>
Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Reviewed-by: Benni Mack <benni@typo3.org>
Reviewed-by: Andreas Fernandez <a.fernandez@scripting-base.de>