Respect direct URLs in puffin installer (#345)

We now write the `direct_url.json` when installing, and _skip_
installing if we find a package installed via the direct URL that the
user is requesting.

A lot of TODOs, especially around cleaning up the `Source` abstraction
and its relationship to `DirectUrl`. I'm gonna keep working on these
today, but this works and makes the requirements clear.

Closes #332.

crates/puffin-resolver/src/distribution/source_distribution.rs

@@ -146,6 +146,11 @@ impl<'a, T: BuildContext> SourceDistributionFetcher<'a, T> {
             return Ok(None);
         };
 
+        // If the commit already contains a complete SHA, short-circuit.
+        if git.precise().is_some() {
+            return Ok(None);
+        }
+
         // Fetch the precise SHA of the Git reference (which could be a branch, a tag, a partial
         // commit, etc.).
         let dir = self.0.cache().join(GIT_CACHE);