[BUGFIX] Fix core update if core location exists

If multiple installations are symlinked to the same core location,
the core updater fails on the second update because the new version
already exists.

Check for this situation and skip unneeded tasks during the update
process.

Change-Id: I4c3c11f0e45c240e15913d5de5a3156ef5ad0aa6
Resolves: #58009
Releases: master, 6.2
Reviewed-on: http://review.typo3.org/36360


Reviewed-by: Mathias Schreiber <mathias.schreiber@wmdb.de>
Tested-by: Mathias Schreiber <mathias.schreiber@wmdb.de>
Reviewed-by: Andreas Fernandez <andreas.fernandez@aspedia.de>
Reviewed-by: Stefan Neufeind <typo3.neufeind@speedpartner.de>
Tested-by: Stefan Neufeind <typo3.neufeind@speedpartner.de>