Use `iter_files` instead of `str(Path(...)` in image dataset
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`iter_archive` is about to support ZIP archives. I think we should use this no ?

see #3347 https://github.com/huggingface/datasets/pull/3379 I was interested in the support for isfile/dir in remote.

Anyway, `iter_files` will be available for community users. I'm not a big fan of having two functions that do the same thing. What do you think ? They do not do the same thing:
- One iterates over files in a directory
- The other I guess will iterate over the members of an archive Makes sense ! Sounds good then - sorry for my misunderstanding

Note that `iter_archive` will be more performant for data streaming that `iter_files` thanks to the buffering so maybe in the future we can `iter_archive` for some of these datasets Yes, @lhoestq I agree with you: once `iter_archive` supports zip files, it will be more suitable than `iter_files` for these 2 datasets.

Anyway, this PR also implements `isfile`/`isdir` in streaming mode, besides fixing `iter_files`. And I'm interested in having those in master.

Maybe, could we merge this PR into master and take note to refactor the datasets to use `iter_archive` once zip is supported?
Other option could be to split this PR into 2...
Use `iter_files` in the `beans` and the `cats_vs_dogs` dataset scripts as suggested by @albertvillanova.

Additional changes:
* Fix `iter_files` in `MockDownloadManager` (see this [CI error](https://app.circleci.com/pipelines/github/huggingface/datasets/9247/workflows/2657ff8a-b531-4fd9-a9fc-6541a72e8d83/jobs/57028))
* Add support for `os.path.isdir` and `os.path.isfile` in streaming (`os.path.isfile` is needed in `StreamingDownloadManager`'s `iter_files` to make `cats_vs_dogs` streamable)

TODO:
- [ ] add tests for `xisdir` and `xisfile`
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