tugger-debian: honor Acquire-By-Hash: yes setting

Modern Debian repositories set an `Acquire-By-Hash: yes` field
to trigger clients to fetch URLs containing the content digest of the
entity they are fetching. We had support for parsing this field but
weren't keying off of it.

This commit implements functionality for resolving the `by-hash` path
for a release file entry and automatically using it if the release
file says to use acquire-by-hash mode.

Pre-existing tests should now fetch teh by-hash URLs. The tests
pass, indicating this functionality apparently works.

tugger-debian/src/repository/http.rs

@@ -280,13 +280,18 @@ impl<'client> HttpReleaseClient<'client> {
                 is_installer,
             )
             .ok_or(HttpError::PackagesIndicesEntryNotFound)?;
-        let path = entry.entry.path;
+
+        let path = if self.release.acquire_by_hash().unwrap_or_default() {
+            entry.entry.by_hash_path()
+        } else {
+            entry.entry.path.to_string()
+        };
 
         // TODO perform digest verification.
         // TODO make this stream output.
 
         let mut reader = ControlParagraphAsyncReader::new(futures::io::BufReader::new(
-            self.get_path_reader(path, entry.compression).await?,
+            self.get_path_reader(&path, entry.compression).await?,
         ));
 
         let mut res = BinaryPackageList::default();

tugger-debian/src/repository/release.rs

@@ -119,6 +119,15 @@ impl<'a> ReleaseFileDigest<'a> {
             Self::Sha256(_) => ChecksumType::Sha256.field_name(),
         }
     }
+
+    /// Obtain the tracked digest value.
+    pub fn digest(&self) -> &'a str {
+        match self {
+            Self::Md5(v) => v,
+            Self::Sha1(v) => v,
+            Self::Sha256(v) => v,
+        }
+    }
 }
 
 /// An entry for a file in a parsed `Release` file.
@@ -136,6 +145,26 @@ pub struct ReleaseFileEntry<'a> {
     pub size: usize,
 }
 
+impl<'a> ReleaseFileEntry<'a> {
+    /// Obtain the `by-hash` path variant for this entry.
+    pub fn by_hash_path(&self) -> String {
+        if let Some((prefix, _)) = self.path.rsplit_once('/') {
+            format!(
+                "{}/by-hash/{}/{}",
+                prefix,
+                self.digest.field_name(),
+                self.digest.digest()
+            )
+        } else {
+            format!(
+                "by-hash/{}/{}",
+                self.digest.field_name(),
+                self.digest.digest()
+            )
+        }
+    }
+}
+
 /// A type of [ReleaseFileEntry] that describes a `Contents` file.
 #[derive(Clone, Debug, PartialEq)]
 pub struct ContentsFileEntry<'a> {
@@ -608,6 +637,10 @@ mod test {
                 size: 738242,
             }
         );
+        assert_eq!(
+            entries[0].by_hash_path(),
+            "contrib/by-hash/MD5Sum/7fdf4db15250af5368cc52a91e8edbce"
+        );
         assert_eq!(
             entries[1],
             ReleaseFileEntry {
@@ -616,6 +649,10 @@ mod test {
                 size: 57319,
             }
         );
+        assert_eq!(
+            entries[1].by_hash_path(),
+            "contrib/by-hash/MD5Sum/cbd7bc4d3eb517ac2b22f929dfc07b47"
+        );
         assert_eq!(
             entries[599],
             ReleaseFileEntry {
@@ -624,6 +661,10 @@ mod test {
                 size: 80488,
             }
         );
+        assert_eq!(
+            entries[599].by_hash_path(),
+            "non-free/source/by-hash/MD5Sum/e3830f6fc5a946b5a5b46e8277e1d86f"
+        );
 
         assert!(release.iter_index_files(ChecksumType::Sha1).is_none());
 
@@ -642,6 +683,7 @@ mod test {
                 size: 738242,
             }
         );
+        assert_eq!(entries[0].by_hash_path(), "contrib/by-hash/SHA256/3957f28db16e3f28c7b34ae84f1c929c567de6970f3f1b95dac9b498dd80fe63");
         assert_eq!(
             entries[1],
             ReleaseFileEntry {
@@ -652,6 +694,7 @@ mod test {
                 size: 57319,
             }
         );
+        assert_eq!(entries[1].by_hash_path(), "contrib/by-hash/SHA256/3e9a121d599b56c08bc8f144e4830807c77c29d7114316d6984ba54695d3db7b");
         assert_eq!(
             entries[599],
             ReleaseFileEntry {
@@ -662,6 +705,7 @@ mod test {
                 path: "non-free/source/Sources.xz",
             }
         );
+        assert_eq!(entries[599].by_hash_path(), "non-free/source/by-hash/SHA256/30f3f996941badb983141e3b29b2ed5941d28cf81f9b5f600bb48f782d386fc7");
 
         let contents = release
             .iter_components_indices(ChecksumType::Sha256)