PmCheck: Use ConLikeSet to model negative info

In #17911 (closed), Simon recognised many warnings stemming from over-long list unions while coverage checking Cabal's LicenseId module.

This patch introduces a new PmAltConSet type which uses a UniqDSet instead of an association list for ConLikes. For PmLits, it will still use an assocation list, though, because a similar map data structure would entail a lot of busy work.