feature: allow co-maintainers to disown their pkg

Derived off of original work done by Leonidas Spyropoulos
at archlinux/aurweb!503

This revision of that original work finishes off the inconsistencies
mentioned in the original MR and adds a small bit of testing for more
regression checks.

Fixes: #360

Signed-off-by: Kevin Morris <kevr@0cost.org>