Extend dataset builder for streaming in `get_dataset_split_names`
I'm impatient to see if it has an impact on the number of valid datasets for the dataset viewer. For the record, today:

<img width="660" alt="Capture d’écran 2022-02-01 à 14 32 19" src="https://user-images.githubusercontent.com/1676121/151977579-b5a239d9-6662-4aeb-bfd1-eef6b8249991.png">
 This is now available in `datasets` 1.18.3 :) I'm on it https://github.com/huggingface/datasets-preview-backend/issues/130
 The result:
<img width="671" alt="Capture d’écran 2022-02-03 à 23 45 55" src="https://user-images.githubusercontent.com/1676121/152442169-bfdac643-9a00-4901-bfa7-1d60a1679d4b.png">

Not very different. Maybe it fixed issues in the community datasets... But I'm not 100% the two states are comparable (datasets have been created, or updated, meanwhile)
Currently, `get_dataset_split_names` doesn't extend a builder module to support streaming, even though it uses `StreamingDownloadManager` to download data. This PR fixes that.

To test the change, run the following:
```bash
pip install git+https://github.com/huggingface/datasets.git@fix-get_dataset_split_names-streaming
python -c "from datasets import get_dataset_split_names; print(get_dataset_split_names('facebook/multilingual_librispeech', 'german', download_mode='force_redownload', revision='137923f945552c6afdd8b60e4a7b43e3088972c1'))"
```