Draft: meson: fix get_version.py to try the package_version file first
We might have a local git repo that was created from the tarball release, in which case there is a package_version file but no tag from which to extract the version via git describe.
Since the package_version file exists only in tarball releases we should always use that even if we're working in a git repo.
This fixes meson configure in GStreamer's cerbero meta-build system which imports tarballs into local git repos.
Draft for now - needs more testing first.