yum-validator: rewrite priorities messaging

Since instructions to the user must now be grouped according to
overrides and non-overrides for RHSM repos, the `verify_priorities` and
`find_package_conflicts` methods have been more closely coupled in
method `run_priority_checks`. Repo priority changes are now deferred
until after both `verify_priorities` and `find_package_conflicts` have
been run, when appropriate (`_commit_resolved_pris`)