package: Avoid processing duplicated package release multiple times

It might exist cases where multiple versions of a package target the same release object.

For instance a rpm package has one specific version for each distribution release but they can target the same intrinsic version and source package contents are exactly the same.

So avoid downloading and processing a package version if the corresponding extid has already been encountered during the current loading by maintaining a mapping between extids and release swhids.

swh/loader/package/debian/loader.py

@@ -107,7 +107,13 @@ class DebianPackageInfo(BasePackageInfo):
                 f"got {len(dsc_files)}"
             )
 
-        return (EXTID_TYPE, EXTID_VERSION, hash_to_bytes(dsc_files[0].sha256))
+        return (
+            EXTID_TYPE,
+            EXTID_VERSION,
+            hash_to_bytes(
+                dsc_files[0].sha256 or dsc_files[0].sha1 or dsc_files[0].md5sum
+            ),
+        )
 
 
 @attr.s