Use pip.req.parse_requirements to parse requirements file

b/c it knows how to handle various options like `--index-url`, `--extra-index-url`, etc.

Without this, pundler didn't know how to handle it when I had a requirements file with `--index-url http://mycompany.com/index`.

I wasn't sure if using this would allow us to remove additional code from pundler -- haven't gotten a chance to try that yet.

Thought I'd show you where I'm going and see if it's something you might want to do.