fix: Don't panic on lack of conflicts

[epage]

After some refactors to pave the way, this re-does the conflict code to disambiguate the groups an argument is a member of from the groups it conflicts with.

Now we only gather real conflicts and then report all of them (might as well since we have them and we went through work to do that for requireds). Behind the scenes, we gather the potential one-way or direct conflicts and cache them. This let's us both reuse the code and reuse the results for then calculating the reverse conflicts (A conflicts with B, therefore B conflicts with A). By finding the reverse like this, we avoid needing to check the order of conflicts (and invert them) when reporting errors, simplifying the code. This isn't too bad because we were able to reuse our iterating over all of the arguments.

Reusable logic/results for conflicts will hopefully make it easier to address #3077

Fixes #3197