[tf.data] Deprecate `tf.contrib.data` and introduce `tf.data.experimental` to replace it.

This change prepares `tf.data` for TensorFlow 2.0, where `tf.contrib` will no longer exist. It retains the pre-existing endpoints in `tf.contrib.data` with deprecation warnings.

Note there are some exceptions to the move:

* Deprecated symbols in `tf.contrib.data` have not been moved to `tf.data.experimental`, because replacements already exist.
* `tf.contrib.data.LMDBDataset` has not been moved, because we plan to move it to a SIG-maintained repository.
* `tf.contrib.data.assert_element_shape()` has not yet been moved, because it depends on functionality in `tf.contrib`, and it will move in a later change.
* `tf.contrib.data.AUTOTUNE` has not yet been moved, because we have not yet determined how to `tf_export()` a Python integer.
* The stats-related API endpoints have not yet appeared in a released version of TensorFlow, so these are moved to `tf.data.experimental` without retaining an endpoint in `tf.contrib.data`.

In addition, this change includes some build rule and ApiDef refactoring:
* Some of the "//third_party/tensorflow/python:training" dependencies had to be split in order to avoid a circular dependency.
* The `tf.contrib.stateless` ops now have a private core library for the generated wrappers (and accordingly are hidden in their ApiDef) so that `tf.data.experimental.sample_from_datasets()` can depend on them.

PiperOrigin-RevId: 215304249