pull: add --repository to pull from external flat repositories

Add a --repository flag taking the full suite URL of an external flat
repository (e.g. https://pkg.noctalia.dev/deb/resolute/), i.e. one with
no dists/ hierarchy, like apt's exact-path suites ('Suites: resolute/').

The suite name is read from the root Release file (Codename/Suite), the
sources index is fetched from the repository root as Sources.xz/gz/plain,
and package files are resolved against the URL root, ignoring the stanza
Directory field like apt does. As with PPAs, the stanza Vcs-Git is never
used for external repositories, so the source always comes from the
repository itself.

Also make the sources index parser detect compression by magic bytes
(gz/xz/plain) instead of assuming gzip, and fix extraction of archives
with './'-prefixed entries, which previously aborted and are now
extracted in place instead of being relocated.

src/build/mod.rs

@@ -1492,6 +1492,7 @@ Provides: virtual-thing (= 2.0), plain-virtual
                 Some(dist),
                 None,
                 None,
+                None,
             ))
             .unwrap_or_else(|e| panic!("package lookup failed for {}: {}", package, e));
         rt.block_on(crate::pull::pull(