Prefer split patterns from directories over split patterns from filenames
_The documentation is not available anymore as the PR was closed or merged._ Can we merge this one since the issue this PR fixes was reported for the second time? I also think we don't need a test for this simple change. @mariosasko sure! could you please approve it?  Hi there @polinaeterna @mariosasko! I have installed 5.2.3.dev0, which should have this fix. Unfortunately, I am still getting the error:
`ValueError: Unknown split "validation". Should be one of ['train'].` When I call `load_dataset("csv", data_files=files, split=split)`

Any help would be greatly appreciated!
related to https://github.com/huggingface/datasets/issues/4895
