[BUGFIX] Make call to isPackageKeyValid non-static

Function isPackagKeyValid on PackageManeger is not a static function
and must be called on an object instance.

Resolves: #59619
Releases: 6.3, 6.2
Change-Id: I03b068db00ccdf76b6e8658721e630b7de6ebfd8
Reviewed-on: http://review.typo3.org/31778
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>