Excessive number of datamodules are declared because of folds

Despite our current efforts removing rgb/grayscale variants, we still have a lot of declared datamodules, what makes it impractical for maintenance.

Ideally, all datamodules for a single database should be concentrated on the same Python module, and referenced from pyproject.toml. To do so, we must implement lazy (cached) properties on DatabaseSplit instances for JSON and CSV split reading.