offline mode

[zkat]

Much like NPM, we should support an "offline" mode that errors out if a package isn't already in the cache.

This can be implemented by a combination of three changes:

1. Add support for passing in a CacheMode to oro-client for metadata requests (no need to worry about it for tarballs)
2. Add an option to nassun::Package::extract_to_dir_inner such that if it's true, it will only try to do self.extract_from_cache instead of all the fallback logic.
3. Expose an --offline global flag (or maybe it should be in ApplyArgs?) and thread it through to the right places.

The rest is just making sure the error messages are good.

Overall, this shouldn't be a very challenging task but it does involve touching a good number of places in the code.

[hibachrach]

I'm gonna give this a shot

[zkat]

All yours! :)

[zkat]

heya! Just checking in. You seem to have gotten pretty far. Are you still interested in getting a PR set up for this? Anything I can help with?

[hibachrach]

Hi! Sorry--lots of not great personal life stuff has happened and so it's probably best for someone to pick up with this WIP (or start over if they prefer). If you're okay waiting I may be able to get back to this in a couple months. Again, really sorry!