[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: Icb34e5c4aa12531fd03217884f58629d6fa1e6ab
Reviewed-on: http://review.typo3.org/16173
Reviewed-by: Helmut Hummel
Tested-by: Helmut Hummel