Set builder name from module instead of class
Now the builder name attribute is set from from the builder class name.

This PR sets the builder name attribute from the module name instead. Some motivating reasons:
- The dataset ID is relevant and unique among all datasets and this is directly related to the repository name, i.e., the name of the directory containing the dataset
- The name of the module (i.e. the file containing the loading loading script) is already relevant for loading: it must have the same name as its containing directory (related to the dataset ID), as we search for it using its directory name
- On the other hand, the name of the builder class is not relevant for loading: in our code, we just search for a class which is subclass of `DatasetBuilder` (independently of its name). We do not put any constraint on the naming of the builder class and indeed it can have a name completely different from its module/direcotry/dataset_id

IMO it makes more sense to align the caching directory name with the dataset_id/directory/module name instead of the builder class name.

Fix #4381.
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