[CLEANUP] Extension Manager code

* Add missing annotations
* Shorten unnecessary namespace annotations

Resolves: #57861
Releases: 6.2
Change-Id: Ic8e24e76e3ccc09122dc0a288f1e0b538685c478
Reviewed-on: https://review.typo3.org/29431
Reviewed-by: Oliver Klee
Reviewed-by: Alexander Opitz
Tested-by: Alexander Opitz
Reviewed-by: Stefan Neufeind
Tested-by: Stefan Neufeind