'Checks if this package is invalid due to having a tag set (this is Ok)
but specifying subpackage names without specifying the main package
name.
Specifying subpackages without a main package name is only valid when
a build is used instead of a tag.
:return: True if this is invalid and False if it\'s valid'

def _is_invalid_subpackages_but_no_main_package(self):

    return (self.tag and self.subpackages and (not self.package))
