Fix data_files when passing data_dir
This code should not return empty data files

```python
from datasets import load_dataset_builder

revision = "3d406e70bc21c3ca92a9a229b4c6fc3ed88279fd"
b = load_dataset_builder("bigcode/the-stack-v2-dedup", data_dir="data/Dockerfile", revision=revision)
print(b.config.data_files)
```

Previously it would return no data files because it would apply the YAML `data_files: data/**/train-*` pattern to this directory

cc @anton-l 