Add support for relative URLs in simple metadata responses (#721)

## Summary

This PR adds support for relative URLs in the simple JSON responses. We
already support relative URLs for HTML responses, but the handling has
been consolidated between the two. Similar to index URLs, we now store
the base alongside the metadata, and use the base when resolving the
URL.

Closes #455.

## Test Plan

`cargo test` (to test HTML indexes). Separately, I also ran `cargo run
-p puffin-cli -- pip-compile requirements.in -n
--index-url=http://localhost:3141/packages/pypi/+simple` on the
`zb/relative` branch with `packse` running, and forced both HTML and
JSON by limiting the `accept` header.

crates/puffin-resolver/src/pins.rs

@@ -2,6 +2,7 @@ use rustc_hash::FxHashMap;
 
 use distribution_types::{File, IndexUrl};
 use puffin_normalize::PackageName;
+use pypi_types::BaseUrl;
 
 use crate::candidate_selector::Candidate;
 
@@ -10,14 +11,20 @@ use crate::candidate_selector::Candidate;
 /// 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)>>);
+pub(crate) struct FilePins(
+    FxHashMap<PackageName, FxHashMap<pep440_rs::Version, (IndexUrl, BaseUrl, File)>>,
+);
 
 impl FilePins {
     /// Pin a candidate package.
-    pub(crate) fn insert(&mut self, candidate: &Candidate, index: &IndexUrl) {
+    pub(crate) fn insert(&mut self, candidate: &Candidate, index: &IndexUrl, base: &BaseUrl) {
         self.0.entry(candidate.name().clone()).or_default().insert(
             candidate.version().clone().into(),
-            (index.clone(), candidate.install().clone().into()),
+            (
+                index.clone(),
+                base.clone(),
+                candidate.install().clone().into(),
+            ),
         );
     }
 
@@ -26,7 +33,7 @@ impl FilePins {
         &self,
         name: &PackageName,
         version: &pep440_rs::Version,
-    ) -> Option<&(IndexUrl, File)> {
+    ) -> Option<&(IndexUrl, BaseUrl, File)> {
         self.0.get(name)?.get(version)
     }
 }