Refactor package manager discovery to allow priorities

[happz]

This has been reported in #2858, dnf was detected instead of rpm-ostree in CoreOS-like image. This was caused by now undefined order of package manager discovery, depending on the order in which plugins were discovered.

Thsi patch adds priorities for package manager families to let them control preferred package manager when multiple were discovered. And a test is added to guard this behavior.

Pull Request Checklist

• [x] implement the feature
• [x] extend the test coverage

[happz]

/packit test

[lukaszachy]

/tests/test/check/avc failed on rawhide (/plans/provision/local), but let's not block this on that.