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/puffin-cli/src/commands/freeze.rs

@@ -1,4 +1,6 @@
 use anyhow::Result;
+use distribution_types::Metadata;
+use itertools::Itertools;
 use tracing::debug;
 
 use platform_host::Platform;
@@ -21,7 +23,10 @@ pub(crate) fn freeze(cache: &Cache, _printer: Printer) -> Result<ExitStatus> {
 
     // Build the installed index.
     let site_packages = SitePackages::from_executable(&python)?;
-    for dist in site_packages.distributions() {
+    for dist in site_packages
+        .iter()
+        .sorted_unstable_by(|a, b| a.name().cmp(b.name()))
+    {
         #[allow(clippy::print_stdout)]
         {
             println!("{dist}");

crates/puffin-cli/src/commands/pip_uninstall.rs

@@ -78,11 +78,7 @@ pub(crate) async fn pip_uninstall(
 
         // Identify all editables that are installed.
         for editable in &editables {
-            if let Some(distribution) = site_packages
-                .editables()
-                .find(|(_dist, url, _dir_info)| url == editable)
-                .map(|(dist, _url, _dir_info)| dist)
-            {
+            if let Some(distribution) = site_packages.get_editable(editable) {
                 distributions.push(distribution);
             } else {
                 writeln!(