## Summary
This PR enables the proof-of-concept resolver to backtrack by way of
using the `pubgrub-rs` crate.
Rather than using PubGrub as a _framework_ (implementing the
`DependencyProvider` trait, letting PubGrub call us), I've instead
copied over PubGrub's primary solver hook (which is only ~100 lines or
so) and modified it for our purposes (e.g., made it async).
There's a lot to improve here, but it's a start that will let us
understand PubGrub's appropriateness for this problem space. A few
observations:
- In simple cases, the resolver is slower than our current (naive)
resolver. I think it's just that the pipelining isn't as efficient as in
the naive case, where we can just stream package and version fetches
concurrently without any bottlenecks.
- A lot of the code here relates to bridging PubGrub with our own
abstractions -- so we need a `PubGrubPackage`, a `PubGrubVersion`, etc.
We can always restore these from history, but right now, it feels a lot
more productive to just hit PyPI directly for our integration tests,
since we don't have to spend time figuring out mocks.
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.
Charlie Marsh