Amid a tragically divided populace, one thing holds us together as a nation right now: the near-unanimous spite regarding a couple of flops, “Megalopolis” from Francis Ford Coppola and “Joker: Folie à Deux,” the “Joker” sequel starring Joaquin Phoenix and Lady Gaga. Decades in the making, writer-director Coppola’s passion project arrived as an alien incursion from a more forgiving era in American film and filmgoing.