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/deb/mod.rs

@@ -383,7 +383,7 @@ mod tests {
 
         log::info!("Pulling package {} from {}...", package, series);
         let package_info =
-            crate::package_info::lookup(package, None, Some(series), "", dist, None, None)
+            crate::package_info::lookup(package, None, Some(series), "", dist, None, None, None)
                 .await
                 .expect("Cannot lookup package information");
         crate::pull::pull(&package_info, Some(cwd), None, true)