[BUGFIX] Resolve relative file path during copy correctly

In composer mode several directories are arranged outside the
public root-path. When trying to copy to those folders, the
public root-path is prefixed due an invalid check which is
too strict. The expectation is to use public root-path
(PATH_site) when resolving relative paths here.

Resolves: #86407
Releases: master
Change-Id: I666b08311e06289c64943518682ab4a970b35a2b
Reviewed-on: https://review.typo3.org/58415


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Susanne Moog <susanne.moog@typo3.org>
Tested-by: Susanne Moog <susanne.moog@typo3.org>
Reviewed-by: Frank Naegler <frank.naegler@typo3.org>
Tested-by: Frank Naegler <frank.naegler@typo3.org>