Resolve data_files by split name
This issue is about discussing the default behavior when someone loads a dataset that consists in data files. For example:
```python
load_dataset("lhoestq/demo1")
```
should return two splits "train" and "test" since the dataset repostiory is like
```
data/
├── train.csv
└── test.csv
```
Currently it returns only one split "train" which contains the data of both files


I started playing with this idea on this branch btw: `resolve-data_files-by-split-name`
Basically the idea is that if you named you data files after split names then the default pattern is
```python
{
    "train": ["*train*"],
    "test": ["*test*"],
    "validation": ["*dev*", "valid"],
}
```
otherwise it's
```python
{
    "train": ["*"]
}
```

Let me know what you think !
cc @albertvillanova @LysandreJik @vblagoje 