DuckPAN: store and parse module list once.

Rather than redownloading a couple (or more) times in a row and
re-parsing, just parse it once per run.

- Eliminates duckpan_packages as an option, but that seemed silly.
- File is mirrored instead of gotten and stored, may make things very
  slightly faster on slow connections.
- File is parsed just once per application run.

Discovered this little bit of insanity while looking for a package version-pinning solution.
:+1: