[BUGFIX] Make overlaid mountpoint pages avoid redirects

If a MountPoint page was overlaid with the Mounted page,
a redirect was triggered. This happened because the slugs
were not identical, so the Mounted Page was not in the
list of the slug candidates.

The behaviour is now resolved, as the Mounted Page is
now added to the candidates and its slug is replaced
by the original slug of the MountPoint page.

Resolves: #90307
Releases: master, 9.5
Change-Id: I0a5bcc320b92e8a2e99ace49f2f84d56cd0b620b
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/63162


Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Daniel Sattler <sattler@b13.de>
Tested-by: Daniel Goerz <daniel.goerz@posteo.de>
Reviewed-by: Daniel Sattler <sattler@b13.de>
Reviewed-by: Daniel Goerz <daniel.goerz@posteo.de>