Draft: meson: fix get_version.py to try the package_version file first

Open Tim-Philipp Müller requested to merge tpm/opus:meson-get-version-fixup into master

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.

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