[WIP] Return local paths to Common Voice
Fixes https://github.com/huggingface/datasets/issues/3663

This is a proposed way of returning the old local file-based generator while keeping the new streaming generator intact.

TODO:
- [ ] brainstorm a bit more on https://github.com/huggingface/datasets/issues/3663 to see if we can do better
- [ ] refactor the heck out of this PR to avoid completely copying the logic between the two generators