Get an all-lowercase version of the requirement's name.

def _key_from_req(req):
    """Get an all-lowercase version of the requirement's name."""
    if hasattr(req, 'key'):
        # from pkg_resources, such as installed dists for pip-sync
        key = req.key
    else:
        # from packaging, such as install requirements from requirements.txt
        key = req.name

    key = key.replace('_', '-').lower()
    return key