Re-add download_manager module in utils
https://github.com/huggingface/datasets/pull/4384 moved `datasets.utils.download_manager` to `datasets.download.download_manager`

This breaks `evaluate` which imports `DownloadMode` from `datasets.utils.download_manager` 

This PR re-adds `datasets.utils.download_manager` without circular imports.

We could also show a message that says that accessing it is deprecated, but I think we can do this in a subsequent PR, and just focus on doing a patch release for now