Remove redirects from the resolver (#2792)
## Summary
Rather than storing the `redirects` on the resolver, this PR just
re-uses the "convert this URL to precise" logic when we convert to a
`Resolution` after-the-fact. I think this is a lot simpler: it removes
state from the resolver, and simplifies a lot of the hooks around
distribution fetching (e.g., `get_or_build_wheel_metadata` no longer
returns `(Metadata23, Option<Url>)`).
