debian: Handle invalid version string when parsing changelog

Some packages can have an invalid version string in their changelog which raises a ValueError when attempting to parse it in the get_intrinsic_package_metadata method of the loader.

As a consequence each package release with such bogus entry in the changelog was discarded from the snapshot created by the loader.

So prefer to get the raw string version instead of parsing it to workaround that issue.

Fix #1493 (closed) and SWH-LOADER-CORE-10W.