Improve dependency tracking

- Add a `.metadata-format-changed-since` file which is a dependency for
  metadata generation. The hope is that this file will change when·ever
  the metadata format does, forcing a rebuild of stale metadata.

- Add `PARSERLIBS` and `TRANSFORMLIBS` (and `EXTRA⸺` variants) to
  enable rebuilds on changes to files that parsers or transforms depend
  on.