Maintain pool repositories after their source project is moved

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In !21339 (merged) we're allowing one pool repository per project, per shard.

When the source project moves shard, the existing pool repository will remain in place for all existing forks to be able to fetch the deduped objects. But no new objects will be added to the poolrepository.

We could still be deduping shared objects from the source project so we have them once per shard.