Pack CWL into a single document for submission.

def _pack_cwl(unpacked_cwl):
    """Pack CWL into a single document for submission.
    """
    out_file = "%s-pack%s" % os.path.splitext(unpacked_cwl)
    cmd = "cwltool --pack {unpacked_cwl} > {out_file}"
    _run_tool(cmd.format(**locals()))
    return out_file