[CLEANUP] Cleanup ExtensionManagementUtility

Methods in this class are called a lot and are in need of some cleanup.
* return early
* reduce method and function calls
* update doc comments
* added tests with data provider for Method getCN

Change-Id: I5dcab647fc3da078b5f29beb57fecbf7c98bcaa5
Resolves: #55122
Releases: 6.2
Reviewed-on: https://review.typo3.org/27019
Reviewed-by: Andreas Wolf
Tested-by: Andreas Wolf