Warn if package list contains part, but not all, of a stable group

[thesamesam]

https://bugs.gentoo.org/939741 could've been avoided if NATTkA either warned or required an override (e.g. a special flag) if packages in the package list form part of a package list but are missing other candidates that have StableRequest in a group.

Or maybe just at the very least we should post a message saying that "XYZ is part of a stable group, consider checking it?"

See also #87.