Migrate resolver proof-of-concept to PubGrub (#97)

## 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.

crates/puffin-resolver/src/error.rs

@@ -2,6 +2,9 @@ use thiserror::Error;
 
 use pep508_rs::Requirement;
 
+use crate::pubgrub::package::PubGrubPackage;
+use crate::pubgrub::version::PubGrubVersion;
+
 #[derive(Error, Debug)]
 pub enum ResolveError {
     #[error("Failed to find a version of {0} that satisfies the requirement")]
@@ -12,6 +15,9 @@ pub enum ResolveError {
 
     #[error(transparent)]
     TrySend(#[from] futures::channel::mpsc::SendError),
+
+    #[error(transparent)]
+    PubGrub(#[from] pubgrub::error::PubGrubError<PubGrubPackage, PubGrubVersion>),
 }
 
 impl<T> From<futures::channel::mpsc::TrySendError<T>> for ResolveError {