message: format: add --api-prefix CLI option
  Needed for checking format when embedded in a parent project.
diff: @@ -94,7 +94,7 @@ def checkProtobufExternalDeps(file_path):
  def isApiFile(file_path):
  - return file_path.startswith('./api/')
  + return file_path.startswith(args.api_prefix)
  def isBuildFile(file_path):
  @@ -291,6 +291,7 @@ if __name__ == "__main__":
  parser.add_argument('--add-excluded-prefixes', type=str, nargs="+", help="exclude additional prefixes.")
  parser.add_argument('-j', '--num-workers', type=int, default=multiprocessing.cpu_count(),
  help="number of worker processes to use; defaults to one per core.")
  + parser.add_argument('--api-prefix', type=str, default='./api/', help="path of the API tree")
  args = parser.parse_args()
  operation_type = args.operation_type
  