Given a go2obj containing key GO IDs, fill with all alternate GO IDs.

def fill_altgoids(go2obj):
    """Given a go2obj containing key GO IDs, fill with all alternate GO IDs."""
    alt2obj = {altgo:goobj for goobj in go2obj.values() for altgo in goobj.alt_ids}
    for goid, goobj in alt2obj.items():
        go2obj[goid] = goobj