Overwrite legacy default config name in `dataset_infos.json` in packaged datasets
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/datasets/pr_6231). All of your documentation changes will be reflected on that endpoint. realized that this pr is still not merged, @lhoestq maybe you can take a look at it?  I think https://github.com/huggingface/datasets/pull/6218 fixed the issue (a bit differently though) ah actually nope, let me check @lhoestq yeah the pr you're referencing doesn't fix the problem when two semantically analogous configs occur in datasets_info.json, i suggest to rewrite the legacy one if it exists during .push_to_hub Only the old versions of `datasets` use the JSON file over the README and they can only load one config so the name doesn't really matter.

That's why I chose to load the info from the JSON no matter the name (no check to see if it's "username--dataset_name") in my previous PR.

I think you can remove the old info without even checking the name. In this case maybe no need to update load.py  (also minor: not checking the name makes it more robust to dataset renaming) @lhoestq okay makes sense... so you think it's not a problem that in some cases we might end up with `dataset_infos.json` having two keys in it? > @lhoestq okay makes sense... so you think it's not a problem that in some cases we might end up with dataset_infos.json having two keys in it?

Ideally they should have only one config no ? Since old versions of `datasets` simply load the first config in the JSON.
We can overwrite it with the new default one (and no matter the name of the outdated config in the JSON)


Currently if we push data as default config with `.push_to_hub` to a repo that has a legacy `dataset_infos.json` file containing a legacy default config name like `{username}--{dataset_name}`, new key `"default"` is added to `dataset_infos.json` along with the legacy one. I think the legacy one should be dropped in this case.

Also, in `load.py` I suggest to check if a legacy config name is indeed a legacy config name because after this fix it might not be the case (this check was first introduced in https://github.com/huggingface/datasets/pull/6218)