Rbarlow 920322

In [0], we realized that we didn't correctly handle the import of package groups that are part of package categories. This pull requests addresses the issue by moving the code that copies package group units and changes their repo_id from import_units() to _import_pkg_group_unit(). This way, the caller of _import_pkg_group_unit() is not responsible for calling save_unit(), and is therefore off the hook for said repo_id alterations.

I did not alter some other types that were doing the same behavior as was fixed here, as they were out of the scope of this ticket and we have not received any bug reports about them to date. This code is fragile, so I decided to make as few changes as possible to avoid breaking other functionality.

[0] https://bugzilla.redhat.com/show_bug.cgi?id=920322