SKUs should only be unique on a per-store basis

It seems to me this may also need a validator to ensure you cannot change the store ID of the product to that of another store that already contains the same SKU.

The UI assumption is that you will never be able to change the store_id, just like you can't change the bundle.

On the code level, we should check if core does anything to prevent bundle changes, and follow its lead.

Reverted in https://github.com/commerceguys/commerce/commit/4bfdcd6ef875aaf4656bdafb..., as a part of preparing for the new product architecture (which will allow a product to belong to multiple stores).

I've done some investigating, and other ecommerce systems (which allow multistore and products in multiple stores, for example Magento) keep the sku validation global. Sites can always use a sku prefix to get around this in certain cases.