Simplify `Resolution` type (#5301)
Looking at how to merge identical forks, i found that the `Resolution`
can be simplified by treating it as a nodes and edges store (plus pins,
they are separate since they are per name-version, not per
(virtual-)package-version). This should also make #5294 more apparent,
which i didn't touch here.
I additionally added some doc comments to the `Resolution` types.
