Expose import-parsing and other low-level parsing tools for other tools to use

[benjaminjkraft]

Webapp's chase_python_imports script ends up duplicating a lot of work in slicker -- understanding what imports mean, finding string references, etc. It would be nice to share a lot of that code! A lot of it is already factored out in slicker so in principle this isn't too hard, but there will inevitably be some differences to handle.