[BUGFIX] Fix namespaces in extension scanner files

Namespaces in the extension scanner files must
not contain a leading backslash.

Resolves: #97547
Releases: main, 11.5
Change-Id: Idb608b3e02c8901972a068c531ee681e8b2dd49e
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/74512


Tested-by: core-ci <typo3@b13.com>
Tested-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Oliver Bartsch <bo@cedev.de>