Fix ImageFolder with parameters drop_metadata=True and drop_labels=False (when metadata.jsonl is present)
Will fix #4621 

ImageFolder raises `KeyError: 'label'` with params `drop_metadata=True` and `drop_labels=False` (if there is at least one metadata.jsonl file a data directory). This happens because metadata files are collected inside `analyze()` function regardless of `drop_metadata` value. And then the following condition doesn't pass: https://github.com/huggingface/datasets/blob/master/src/datasets/packaged_modules/imagefolder/imagefolder.py#L167

So I suggest to double check it inside `analyze()` not to collect metadata files if they are not needed. (and labels too, to be consistent)

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Also, I added a test to check if labels are inferred correctly from directories names in general (because we didn't have it) :)