build: lock down `data_compat` and `dataclass_compat` (#3504)
build: lock down `data_compat` and `dataclass_compat` (#3504)

Summary:
These should both have been internal from the beginning, but team policy
when `data_compat` was created was to be public by default, and we
followed precedent with `dataclass_compat`. There are no remaining
Google-internal non-TensorBoard dependents.

Test Plan:
Googlers, see <http://cl/305916851> for test sync.

wchargin-branch: vis-lock-down-compats
tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard