Add an editable index to the site-packages registry (#671)

This PR modifies `SitePackages` to store all distributions in a flat
vector, and maintain two indexes (hash maps) from "per-element data for
an element in the vector" to "index of that element". This enables us to
maintain a map on both package name and editable URL.

crates/distribution-types/src/installed.rs

@@ -3,6 +3,7 @@ use std::str::FromStr;
 
 use anyhow::{anyhow, Context, Result};
 use fs_err as fs;
+use url::Url;
 
 use pep440_rs::Version;
 use puffin_normalize::PackageName;
@@ -140,4 +141,15 @@ impl InstalledDist {
         Metadata21::parse(&contents)
             .with_context(|| format!("Failed to parse METADATA file at: {}", path.display()))
     }
+
+    /// Return the [`Url`] of the distribution, if it is editable.
+    pub fn editable(&self) -> Option<&Url> {
+        match self {
+            Self::Url(InstalledDirectUrlDist {
+                url: DirectUrl::LocalDirectory { url, dir_info },
+                ..
+            }) if dir_info.editable == Some(true) => Some(url),
+            _ => None,
+        }
+    }
 }