Add support for downgrade and upgrade of packages with different heuristics

[geaaru]

Currently luet upgrade upgrades a system to the latest available packages, not taking care of downgrades of any sort. E.g. if a package is downgraded from a repository, the client will refuse to downgrade to that version, but will prefer the local installed one.

The idea is to implement a command like luet upgrade --purge/--sync for:

• [ ] permit downgrade of packages related to a removed repository (example: foo-1.0@removed-repo and foo-0.2.0@active-repo)

• [ ] permit downgrade of packages no more available from a specific repository for an error server side

• [x] permit to upgrade of packages with the same version but with different build date (an option could be --sync-by-date)

[mudler]

This is the behavior of luet upgrade --universe, we have just to add more tests to it and promote its usage