Extract requirements file parsing from line handling
Extract requirements file parsing from line handling

By using a separate entity to parse lines and recurse into other
requirements files, we can more easily use different strategies for
handling the incoming requirements info.

sbidoul/pip,xavfernandez/pip,pradyunsg/pip,pradyunsg/pip,pfmoore/pip,pypa/pip,sbidoul/pip,xavfernandez/pip,xavfernandez/pip,pypa/pip,pfmoore/pip