Parse pip requirements file.

def parse_pip_file(path):
    """Parse pip requirements file."""
    # requirement lines sorted by importance
    # also collect other pip commands
    rdev = dict()
    rnormal = []
    stuff = []

    try:
        with open(path) as f:
            for line in f:
                line = line.strip()

                # see https://pip.readthedocs.org/en/1.1/requirements.html
                if line.startswith('-e'):
                    # devel requirement
                    splitted = line.split('#egg=')
                    rdev[splitted[1].lower()] = line

                elif line.startswith('-r'):
                    # recursive file command
                    splitted = re.split('-r\\s+', line)
                    subrdev, subrnormal, substuff = parse_pip_file(splitted[1])
                    for k, v in subrdev.iteritems():
                        if k not in rdev:
                            rdev[k] = v
                    rnormal.extend(subrnormal)
                    result.extend(substuff)

                elif line.startswith('-'):
                    # another special command we don't recognize
                    stuff.append(line)

                else:
                    # ordenary requirement, similary to them used in setup.py
                    rnormal.append(line)
    except IOError:
        print(
            'Warning: could not parse requirements file "{}"!',
            file=sys.stderr
        )

    return rdev, rnormal, stuff