List all versions of a package

        Along with the version, the caller also receives the file list with all
        the available formats.

def package_releases(self, package, url_fmt=lambda u: u):
        """List all versions of a package

        Along with the version, the caller also receives the file list with all
        the available formats.
        """
        return [{
            'name': package,
            'version': version,
            'urls': [self.get_urlhash(f, url_fmt) for f in files]
        } for version, files in self.storage.get(package, {}).items()]