fix: files counted twice in inferred structure
Files were counted twice in a structure like:
```
my_dataset_local_path/
├── README.md
└── data/
    ├── train/
    │   ├── shard_0.csv
    │   ├── shard_1.csv
    │   ├── shard_2.csv
    │   └── shard_3.csv
    └── valid/
        ├── shard_0.csv
        └── shard_1.csv
```

The reason is that they were matching both `*train*/*` and `*train*/**/*`.

This PR fixes it. @lhoestq 