We should always create a generic purl otherwise if we cannot infer it as in pkg:generic/lxc-master?download_url=https://salsa.debian.org/lxc-team/lxc/-/archive/master/lxc-master.tar.gz ... see https://github.com/package-url/purl-spec/blob/master/PURL-TYPES.rst#generic

This helps a lot downstream users such as ScanCode to get always something and at the very least a generic purl.