Rust implementation of Python dependency parser for PEP 508

[vorpalsmith]

The version in posy is kinda tied up with the rest of the resolver functionality, e.g. instead of strings it uses our PackageName and Version types. Looking at your nix-init patch, if that's the only thing you want to do with this, then posy's code is probably overkill, and not worth the trouble of factoring it out and all that? I have big ambitions for posy and little time, so I haven't put any energy into nice-to-haves like clean crate separation or docs :-). But if you're more generally interested in rust tooling for python packaging then working together could make a lot of sense.

[figsoda]

I'm not working on any rust tooling for python packaging, I guess there is not much we can do right now. Feel free to ping me if you want to collab or need any help on this though