load_dataset is case sentitive?
### Describe the bug

load_dataset() function is case sensitive?

### Steps to reproduce the bug

The following two code, get totally different behavior.

1. load_dataset('mbzuai/bactrian-x','en')

2. load_dataset('MBZUAI/Bactrian-X','en')

### Expected behavior

Compare 1 and 2.
1 will download all 52 subsets, shell output:
```Downloading and preparing dataset json/MBZUAI--bactrian-X to xxx```
2 will only download single subset, shell output
```Downloading and preparing dataset bactrian-x/en to xxx```


### Environment info

Python 3.10.11
datasets Version: 2.11.0
Hi @haonan-li , thank you for the report! It seems to be a bug on the [`huggingface_hub`](https://github.com/huggingface/huggingface_hub) site, there is even no such dataset as `mbzuai/bactrian-x` on the Hub.  I opened and [issue](https://github.com/huggingface/huggingface_hub/issues/1453) there. I think `load_dataset("mbzuai/bactrian-x")` shouldn't be loaded at all and raise an error but because of [this fallback](https://github.com/huggingface/datasets/blob/main/src/datasets/load.py#L1194) to packaged loaders when no other options are applicable, it loads the dataset with standard `json` loader instead of the custom loading script.