Mocks out the PyPI client using some checked-in fixtures. The test is
very basic, and I'm not very happy with all the ceremony around the
mocks and such, but it's an interesting experiment at least.
This PR modifies the `install-wheel-rs` (and a few other crates) to get
everything playing nicely. Specifically, CI should pass, and all these
crates now use workspace dependencies between one another.
As part of this change, I split out the wheel name parsing into its own
`wheel-filename` crate, and the compatibility tag parsing into its own
`platform-tags` crate.