[BUGFIX] Respect PATH_site in t3lib_autoloader::getAutoloadCacheIdentifier

Function t3lib_autoloader::getAutoloadCacheIdentifier now respects the constant
PATH_site. This is needed because otherwise a fatal error occures when the
path to webroot changes (e.g. moving site to another webserver)

Change-Id: I9755da82cbf71914d47bc0523bacc8413addcf63
Resolves: #29901
Releases: 4.6
Reviewed-on: http://review.typo3.org/5000
Reviewed-by: Steffen Gebert
Tested-by: Steffen Gebert
Reviewed-by: Helmut Hummel
Tested-by: Helmut Hummel