Fix streaming zip files
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Hi ! I don't fully understand this change @albertvillanova 
The `_extract` method used to return the compound URL that points to the root of the inside of the archive.
This way users can use the usual os.path.join or other functions to point to the relevant files. I don't see why you're using a glob pattern ?
2021-08-16T13:49:26Z
https://github.com/huggingface/datasets/pull/2798#issuecomment-899526629
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MEMBER
This change is to allow this:
```python
data_files = f"https://huggingface.co/datasets/albertvillanova/datasets-tests-compression/resolve/main/sample.zip"
ds = load_dataset("json", split="train", data_files=data_files, streaming=True)
assert isinstance(ds, IterableDataset)
```
Note that in this case the user will not call os.path.join.

Before this PR it gave error because pointing to the root, without any subsequent join, gives error:
```python
fsspec.open("zip://::https://huggingface.co/datasets/albertvillanova/datasets-tests-compression/resolve/main/sample.zip")
```
2021-08-16T14:14:40Z
https://github.com/huggingface/datasets/pull/2798#issuecomment-899546486
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2021-08-16T14:16:50Z
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Currently, streaming remote zip data files gives `FileNotFoundError` message:
```python
data_files = f"https://huggingface.co/datasets/albertvillanova/datasets-tests-compression/resolve/main/sample.zip"
ds = load_dataset("json", split="train", data_files=data_files, streaming=True)
next(iter(ds))
```

This PR fixes it by adding a glob string.

The corresponding test is implemented in PR #2786.