message: Undo BC-breaking change, restore 'import onnx' providing submodules.
diff: @@ -6,6 +6,11 @@ from __future__ import unicode_literals
  from .onnx_ml_pb2 import * # noqa
  from .version import version as __version__ # noqa
  +# Import common subpackages so they're available when you 'import onnx'
  +import onnx.helper # noqa
  +import onnx.checker # noqa
  +import onnx.defs # noqa
  +
  import sys
  def load(obj):
  