Error when loading a metric
## Describe the bug
As reported by @sgugger, after last release, exception is thrown when loading a metric.

## Steps to reproduce the bug
```python
from datasets import load_metric

metric = load_metric("squad_v2")
```

## Actual results
```
FileNotFoundError                         Traceback (most recent call last)
<ipython-input-1-e612a8cab787> in <module>
      1 from datasets import load_metric
----> 2 metric = load_metric("squad_v2")

d:\projects\huggingface\datasets\src\datasets\load.py in load_metric(path, config_name, process_id, num_process, cache_dir, experiment_id, keep_in_memory, download_config, download_mode, revision, script_version, **metric_init_kwargs)
   1336         )
   1337         revision = script_version
-> 1338     metric_module = metric_module_factory(
   1339         path, revision=revision, download_config=download_config, download_mode=download_mode
   1340     ).module_path

d:\projects\huggingface\datasets\src\datasets\load.py in metric_module_factory(path, revision, download_config, download_mode, force_local_path, dynamic_modules_path, **download_kwargs)
   1237                 if not isinstance(e1, FileNotFoundError):
   1238                     raise e1 from None
-> 1239                 raise FileNotFoundError(
   1240                     f"Couldn't find a metric script at {relative_to_absolute_path(combined_path)}. "
   1241                     f"Metric '{path}' doesn't exist on the Hugging Face Hub either."

FileNotFoundError: Couldn't find a metric script at D:\projects\huggingface\datasets\squad_v2\squad_v2.py. Metric 'squad_v2' doesn't exist on the Hugging Face Hub either.
```

