Deduplicate lenient parsing code between version specifiers and
Requirement. Use `warn_once!` since the warnings did show up multiple
times in my code. Fix the macro hygiene in `warn_once!`.
## Summary
This PR unifies the behavior that lived in the resolver's `distribution`
crates with the behaviors that were spread between the various structs
in the installer crate into a single `Fetcher` struct that is intended
to manage all interactions with distributions. Specifically, the
interface of this struct is such that it can access distribution
metadata, download distributions, return those downloads, etc., all with
a common cache.
Overall, this is mostly just DRYing up code that was repeated between
the two crates, and putting it behind a reasonable shared interface.