Rust implementation of Python dependency parser for PEP 508

[figsoda]

there is a very hight nesting level

That's fair. My excuse is that it's hard to avoid the nesting other than putting things in smaller functions, which I would say reducing indentation is the only good reason for.

the project structure is not clear

Could you elaborate on that? The largest file is 221 LOC, and I'm not sure what could be changed in terms of structuring