[Cleanup] Rewrite checksums.py to use importlib_resources

[Conchylicultor]

Currently, https://github.com/tensorflow/datasets/blob/master/tensorflow_datasets/core/download/checksums.py read checksums using relative dir:

datasets/tensorflow_datasets/core/download/checksums.py

Line 28 in 0c308e4

_ROOT_DIR = os.path.normpath(os.path.join(os.path.dirname(__file__), '../..'))

This is hacky when more modern https://docs.python.org/3/library/importlib.html#importlib.resources.files exists with a Pathlib-like interface.

path = importlib_resources.files('tensorflow_datasets') / 'url_checksums'
files = [p for p in path.listdir() if str(p).endswith('txt')]

It would make TFDS work even when Python is executed inside compressed .zip archive.

[iamsdas]

Added the PR