swh.lister.gnu: Add functionality to list all the tarballs for a package.

As discussed in T1389 to ingest all packages using base loader, it need
a list of all the tarballs for a pakage.
Hence modifified lister to recursively list all the tarballs for a
package with their last updated time.

README.md

@@ -16,6 +16,7 @@ following Python modules:
 - `swh.lister.pypi`
 - `swh.lister.npm`
 - `swh.lister.phabricator`
+- `swh.lister.gnu`
 
 Dependencies
 ------------