Get the full dict of maps {map_name: map_class}.

def get_maps():
  """Get the full dict of maps {map_name: map_class}."""
  maps = {}
  for mp in Map.all_subclasses():
    if mp.filename:
      map_name = mp.__name__
      if map_name in maps:
        raise DuplicateMapException("Duplicate map found: " + map_name)
      maps[map_name] = mp
  return maps