Combine fetch and resolve steps in Git resolver (#5886)
## Summary
Whenever we call `resolve`, we immediately call `fetch` after. And in
some cases `resolve` actually calls `fetch` internally. It seems a lot
simpler to just merge these into one method that returns a `Fetch`
(which itself contains the fully-resolved URL).
Closes https://github.com/astral-sh/uv/issues/5876.
