[TASK] Rename ExtensionManager class

[TASK] Rename ExtensionManager class

The class ExtensionManager is actually a utility
to manage extensions. Rename it to better reflect
it's purpose.

Resolves: #42661
Releases: 6.0

Change-Id: I9662c3feef3a94e7ede8fd46b55b2531debad1ab
Reviewed-on: http://review.typo3.org/16172
Reviewed-by: Helmut Hummel
Tested-by: Helmut Hummel