Add tensorflow/python/platform/__init__.py for clearer error msg.

Without this init file, when trying to import tensorflow python code
from the source tree you get this error....

File "tensorflow/tensorflow/python/pywrap_tensorflow.py", line 25, in <module>
  from tensorflow.python.platform import self_check
ImportError: No module named platform

With this init file you get a much clearer error message telling you
not to do this...

File "tensorflow/tensorflow/python/pywrap_tensorflow.py", line 25, in <module>
  from tensorflow.python.platform import self_check
File "tensorflow/tensorflow/python/platform/self_check.py", line 27, in <module>
    raise ImportError("Could not import tensorflow. Do not import tensorflow "
ImportError: Could not import tensorflow. Do not import tensorflow from its source directory; change directory to outside the TensorFlow source tree, and relaunch your Python interpreter from there.
PiperOrigin-RevId: 221662931

tensorflow/python/eager/test.py

@@ -24,6 +24,6 @@ from tensorflow.python.platform.test import *  # pylint: disable=wildcard-import
 
 
 # TODO(akshayka): Do away with this file.
-def main(argv=None):
+def main(argv=None):  # pylint: disable=function-redefined
   _ops.enable_eager_execution()
   _test.main(argv)