[BUGFIX] class_alias in the bootstrap for legacy base classes

The legacy base classes t3lib_div and t3lib_extMgm used
to be available early in the bootstrap.

To enable the usage of the old class names a class alias
needs to be set up right after these classes are included.

Change-Id: I3cfa69d9bbca378015ee5ea3c13a6fac032fe9cc
Fixes: #42574
Resolves: #41122
Releases: 6.0
Reviewed-on: http://review.typo3.org/14776
Reviewed-by: Nicole Cordes
Tested-by: Nicole Cordes
Reviewed-by: Helmut Hummel
Tested-by: Helmut Hummel