Potential improvement for editable installs

I noticed another small difference between `pip-api` and `pip-requirements-parser` which I'd like to bring up.
At the moment, `pip-requirements-parser` relies on an egg fragment being at the end of an editable install in order to infer its name/specifier. Otherwise, the `EditableRequirement`'s `req` member is set to `None`.

`pip-api` has some special logic that checks whether the editable installation points to a directory. If so, it checks that directory for a `setup.py` and attempts to parse the package name and version out of the file. You can find the relevant logic [here](https://github.com/di/pip-api/blob/master/pip_api/_parse_requirements.py#L248).

How do you feel about having this logic in `pip-requirements-parser`? I appreciate that this logic perhaps goes a bit beyond "parsing" but it'd be useful since not all editable requirements have an egg fragment.