[TASK] Bring back t3lib_extMgm class alias

t3lib_extMgm::extPath() is used in ext_autoload.php of lots of third
party extensions. Removing this alias lets the core fatal, resulting
in a stalled lock file in early bootstrap. This gives quite some hassle.
To mitigate, t3lib_extMgm class alias is exceptionally added again.

Resolves: #62852
Releases: master
Change-Id: I309aaae067ec4a430ebb8985554cb6750eb8ec09
Reviewed-on: http://review.typo3.org/34014
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>