No data files duplicates
I added a new DataFilesSet class to disallow duplicate data files.
I also deprecated DataFilesList.

EDIT: actually I might just add drop_duplicates=True to `.from_patterns`

close https://github.com/huggingface/datasets/issues/6259
close https://github.com/huggingface/datasets/issues/6272

TODO:
- [ ] tests
- [ ] preserve data files order