Shift TensorBoard to depend on TensorFlow via `import tensorflow as tf`.

This is in preparation for TensorBoard leaving github/tensorflow/tensorflow
and moving into its own repository.

There is a little trickiness around tf.contrib.tensorboard, which
has the potential to generate circular dependencies because it depends on
tensorboard.plugins, which now depends on tensorflow, which depends on
contrib. I disconnected the only circle at tensorboard.backend.json_util.

Eventually, I hope for tensorboard to have no dependencies on
//tensorflow except for files within tensorboard, and on the
tensorflow_py target.

We need to port or make public a few utility functions (e.g.
tensor_util.MakeNdarray) and solve the contrib circular dep before
that will be the case.

PiperOrigin-RevId: 157288184