[BUGFIX] Avoid fatal error if adding composer package

Adding composer packages to „Packages“ folder will cause a fatal error
This fix prevents the extension manager from handling not legacy packages

Resolves: #55754
Releases: 6.2
Change-Id: I3ac491c8566b14578f5d3a604c63dffc1310d042
Reviewed-on: https://review.typo3.org/27402
Reviewed-by: Thomas Maroschik
Reviewed-by: Marcin Sągol
Tested-by: Thomas Maroschik
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn