Remove internal variable names when moving from sub-workflow to main.

def _extract_from_subworkflow(vs, step):
    """Remove internal variable names when moving from sub-workflow to main.
    """
    substep_ids = set([x.name for x in step.workflow])
    out = []
    for var in vs:
        internal = False
        parts = var["id"].split("/")
        if len(parts) > 1:
            if parts[0] in substep_ids:
                internal = True
        if not internal:
            var.pop("source", None)
            out.append(var)
    return out