Use `huggingface_hub` cache
- use `hf_hub_download()` from `huggingface_hub` for HF files
- `datasets` cache_dir is still used for:
  - caching datasets as Arrow files (that back `Dataset` objects)
  - extracted archives, uncompressed files
  - files downloaded via http (datasets with scripts)
- I removed code that were made for http files (and also the dummy_data / mock_download_manager stuff that happened to rely on them and have been legacy for a while now)