Avoid double-resolving during pip-install (#610)

## Summary

At present, when performing a `pip-install`, we first do a resolution,
then take the set of requirements and basically run them through our
`pip-sync`, which itself includes re-resolving the dependencies to get a
specific `Dist` for each package. (E.g., the set of requirements might
say `flask==3.0.0`, but the installer needs a specific _wheel_ or source
distribution to install.)

This PR removes this second resolution by exposing the set of pinned
packages from the resolution. The main challenge here is that we have an
optimization in the resolver such that we let the resolver read metadata
from an incompatible wheel as long as a source distribution exists for a
given package. This lets us avoid building source distributions in the
resolver under the assumption that we'll be able to install the package
later on, if needed. As such, the resolver now needs to track the
resolution and installation filenames separately.

crates/puffin-resolver/src/pins.rs

@@ -0,0 +1,30 @@
+use crate::candidate_selector::Candidate;
+use puffin_normalize::PackageName;
+use pypi_types::{File, IndexUrl};
+use rustc_hash::FxHashMap;
+
+/// A set of package versions pinned to specific files.
+///
+/// For example, given `Flask==3.0.0`, the [`FilePins`] would contain a mapping from `Flask` to
+/// `3.0.0` to the specific wheel or source distribution archive that was pinned for that version.
+#[derive(Debug, Default)]
+pub(crate) struct FilePins(FxHashMap<PackageName, FxHashMap<pep440_rs::Version, (IndexUrl, File)>>);
+
+impl FilePins {
+    /// Pin a candidate package.
+    pub(crate) fn insert(&mut self, candidate: &Candidate, index: &IndexUrl) {
+        self.0.entry(candidate.name().clone()).or_default().insert(
+            candidate.version().clone().into(),
+            (index.clone(), candidate.install().clone().into()),
+        );
+    }
+
+    /// Return the pinned file for the given package name and version, if it exists.
+    pub(crate) fn get(
+        &self,
+        name: &PackageName,
+        version: &pep440_rs::Version,
+    ) -> Option<&(IndexUrl, File)> {
+        self.0.get(name)?.get(version)
+    }
+}