[TASK] Move ExtensionManager Class part 1

Only move the class to keep track of history

Related: #42661
Releases: 6.0

Change-Id: I9118c41c78493369ccc12ff5443c3faf2261fad4
Reviewed-on: http://review.typo3.org/16162
Reviewed-by: Wouter Wolters
Reviewed-by: Helmut Hummel
Tested-by: Helmut Hummel