Replace tfds pathlib API by `epath.Path`
Replace tfds pathlib API by `epath.Path`

TFDS pathlib API is moving in a separate util. This allow other project to use the pathlib API without having to depend on TFDS

```
from etils import epath

path = epath.Path('gs://some-bucket/file.json')
content = path.read_text()
```

`tfds.core.Path` is an alias of `etils.Path` and `tfds.core.as_path` is kept for now for backward compatibility

Other functions/alias have been removed from TFDS.

PiperOrigin-RevId: 420037254

google-research/meta-dataset