Extend dataset builder for streaming in `get_dataset_split_names`
{'login': 'mariosasko', 'id': 47462742, 'node_id': 'MDQ6VXNlcjQ3NDYyNzQy', 'avatar_url': 'https://avatars.githubusercontent.com/u/47462742?v=4', 'gravatar_id': '', 'url': 'https://api.github.com/users/mariosasko', 'html_url': 'https://github.com/mariosasko', 'followers_url': 'https://api.github.com/users/mariosasko/followers', 'following_url': 'https://api.github.com/users/mariosasko/following{/other_user}', 'gists_url': 'https://api.github.com/users/mariosasko/gists{/gist_id}', 'starred_url': 'https://api.github.com/users/mariosasko/starred{/owner}{/repo}', 'subscriptions_url': 'https://api.github.com/users/mariosasko/subscriptions', 'organizations_url': 'https://api.github.com/users/mariosasko/orgs', 'repos_url': 'https://api.github.com/users/mariosasko/repos', 'events_url': 'https://api.github.com/users/mariosasko/events{/privacy}', 'received_events_url': 'https://api.github.com/users/mariosasko/received_events', 'type': 'User', 'site_admin': False}
[]
[]
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'))"
```
{'url': 'https://api.github.com/repos/huggingface/datasets/issues/3657/reactions', 'total_count': 0, '+1': 0, '-1': 0, 'laugh': 0, 'hooray': 0, 'confused': 0, 'heart': 0, 'rocket': 0, 'eyes': 0}
{'url': 'https://api.github.com/repos/huggingface/datasets/pulls/3657', 'html_url': 'https://github.com/huggingface/datasets/pull/3657', 'diff_url': 'https://github.com/huggingface/datasets/pull/3657.diff', 'patch_url': 'https://github.com/huggingface/datasets/pull/3657.patch', 'merged_at': '2022-02-02T11:22:01Z'}