Retroactively shard dataset into subdatasets

[effigies]

What would you like to see added?

If a dataset becomes too large, it would be good to turn subject directories into submodules. For tagging purposes, we would need to find some way of identifying the old and new commits (grafting?).

At least for datasets with new subjects, one possibility would be to shard new subjects, but new sessions within existing subjects may or may not need to be handled differently.

Derivatives and sourcedata would also be candidates for sharding.

Alternatives

No response

Do you have any interest in helping implement the feature?

No

Additional information / screenshots

No response

[effigies]

Related: datalad/datalad#3554

[yarikoptic]

do you have examples of already existing openneuro datasets and how would you like to split?