Refactor `Resolution` type to retain dependency graph (#9106)
## Summary
This PR should not contain any user-visible changes, but the goal is to
refactor the `Resolution` type to retain a dependency graph. We want to
be able to explain _why_ a given package was excluded on error (see:
https://github.com/astral-sh/uv/issues/8962), which in turn requires
that at install time, we can go back and figure out the dependency
chain. At present, `Resolution` is just a map from package name to
distribution; this PR remodels it as a graph in which each node is a
package, and the edges contain markers plus extras or dependency groups.
