Inconsistent splits generation for datasets without loading script (packaged dataset puts everything into a single split)
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## Describe the bug
Splits for dataset loaders without scripts are prepared inconsistently. I think it might be confusing for users.

## Steps to reproduce the bug
* If you load a packaged datasets from Hub, it infers splits from directory structure / filenames (check out the data [here](https://huggingface.co/datasets/nateraw/test-imagefolder-dataset)):
```python
ds = load_dataset("nateraw/test-imagefolder-dataset")
print(ds)
### Output:
DatasetDict({
    train: Dataset({
        features: ['image', 'label'],
        num_rows: 6
    })
    test: Dataset({
        features: ['image', 'label'],
        num_rows: 4
    })
})
```
* If you do the same from locally stored data specifying only directory path you'll get the same:
```python
ds = load_dataset("/path/to/local/data/test-imagefolder-dataset")
print(ds)
### Output:
DatasetDict({
    train: Dataset({
        features: ['image', 'label'],
        num_rows: 6
    })
    test: Dataset({
        features: ['image', 'label'],
        num_rows: 4
    })
})
```
* However, if you explicitely specify package name (like `imagefolder`, `csv`, `json`), all the data is put into a single split:
```python
ds = load_dataset("imagefolder", data_dir="/path/to/local/data/test-imagefolder-dataset")
print(ds)
### Output:
DatasetDict({
    train: Dataset({
        features: ['image', 'label'],
        num_rows: 10
    })
})
```

## Expected results
For `load_dataset("imagefolder", data_dir="/path/to/local/data/test-imagefolder-dataset")` I expect the same output as of the two first options.
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