Run the single hook specified by resolving all its prerequisites.

def _RunSingleHook(self, hook_cls, executed_set, required=None):
    """Run the single hook specified by resolving all its prerequisites."""
    # If we already ran do nothing.
    if hook_cls in executed_set:
      return

    # Ensure all the pre execution hooks are run.
    for pre_hook in hook_cls.pre:
      self._RunSingleHook(pre_hook, executed_set, required=hook_cls.__name__)

    # Now run this hook.
    cls_instance = hook_cls()
    if required:
      logging.debug("Initializing %s, required by %s", hook_cls.__name__,
                    required)
    else:
      logging.debug("Initializing %s", hook_cls.__name__)

    # Always call the Run hook.
    cls_instance.Run()
    executed_set.add(hook_cls)

    # Only call the RunOnce() hook if not already called.
    if hook_cls not in self.already_run_once:
      cls_instance.RunOnce()
      self.already_run_once.add(hook_cls)