Copy packages check improvements

[Celso Providelo]

The copy-package infrastructure checks have to be extended to cover the following situations:

 1. get_ancestry_candidate: should follow pocket_dependencies and ensure a copy candidate will be the highest version in the context it is distributed (for instance, security copied should be the highest publication across release, updates and security, but not necessarily higher than proposed or backports)

 2. check_archive_conflicts: should also look for conflicts in removed publications, this way we can be sure that source versions are not being reused and causing trouble on client systems.

The former change is a little bit over-the-top for PPAs, we could probably only apply it when copies are targeted to the primary archive.