Generate typing overload for JPackage with string variable argument

Currently mypy complains when JPackage is instantiated with a string variable, e.g.:

error: No overload variant of "JPackage" matches argument type "str"  [call-overload]
note: Possible overload variants:
note:     def JPackage(Literal['cern'], /) -> __module_protocol__
note:     def JPackage(Literal['java'], /) -> __module_protocol__

Adding this additional overload fixes the error.