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/pep508-rs/src/lib.rs

@@ -264,7 +264,10 @@ impl Requirement {
     #[allow(clippy::needless_pass_by_value)]
     #[pyo3(name = "evaluate_markers")]
     pub fn py_evaluate_markers(&self, env: &MarkerEnvironment, extras: Vec<String>) -> bool {
-        self.evaluate_markers(env, &extras)
+        self.evaluate_markers(
+            env,
+            &extras.iter().map(String::as_str).collect::<Vec<&str>>(),
+        )
     }
 
     /// Returns whether the requirement would be satisfied, independent of environment markers, i.e.
@@ -320,12 +323,9 @@ impl Requirement {
     }
 
     /// Returns whether the markers apply for the given environment
-    pub fn evaluate_markers(&self, env: &MarkerEnvironment, extras: &[String]) -> bool {
+    pub fn evaluate_markers(&self, env: &MarkerEnvironment, extras: &[&str]) -> bool {
         if let Some(marker) = &self.marker {
-            marker.evaluate(
-                env,
-                &extras.iter().map(String::as_str).collect::<Vec<&str>>(),
-            )
+            marker.evaluate(env, extras)
         } else {
             true
         }