## 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.
When we go to install a locked `requirements.txt`, if a wheel is already
available in the local cache, and matches the version specifiers, we can
just use it directly without fetching the package metadata. This speeds
up the no-op case by about 33%.
Closes https://github.com/astral-sh/puffin/issues/48.
This PR modifies the PEP 440 and PEP 508 crates to pass CI, primarily by
fixing all lint violations.
We're also now using these crates in the workspace via `path`.
(Previously, we were still fetching them from Cargo.)
This PR copies over the `pep440-rs` crate at commit
`82aa5d4dcbe676b121dc931b0afa09a82de8e3d7` with no modifications.
It won't pass CI, but modifications will intentionally be confined to
later PRs.
