This is an attempt to fix our long-standing problems dealing with maintainer
information. Move the actual maintainer information off of the package model
into a PackageRelation object, which has some flexibility to later represent
more than just maintainership.

This solves multiple problems:
* If a package gets accidentally deleted, so did the maintainer info
* Testing packages have always shown up as orphans
* With split packages, it was easy to miss some of the sub-packages

This commit does not include the deletion of the original maintainer column;
that will come at a later time when I feel more confident that the data was
migrated correctly.

Signed-off-by: Dan McGee <dan@archlinux.org>