Complete Token support for Product SKUs

We've discussed having Token replacement for SKUs with default SKU patterns per product type. This is a good compromise for sites that don't even use SKUs, in that every product can just use the [product:product-id] as the SKU and not have to think about it further. The way it should work is when a product form is saved, any Tokens in the SKU should be replaced so the evaluated string is stored in the database. Each product type should be able to specify a default SKU pattern that is set as the default value of the SKU field on the add form for products of that type.

When the form is submitted, it gets tricky. We need to be able to validate the SKU, which requires checking to ensure that only valid tokens are used, that the resultant SKU only contains valid characters, and that the resultant SKU is unique. The problem is that for new products, we don't have a fully populated $product object to use for Token replacement. We won't have that until the form submits, so during validation we need to fake values on the $product object and then after the product has been saved, update the SKU accordingly. This will be tricky, and my hunch is it will involve saving the SKU pattern with a unique value appended temporarily and then upon save re-loading the product and updating the sku field. :-/

I question whether or not this should be a core feature. On the one hand, it's hardly necessary for the framework. On the other, it's a feature that needs to be so tightly integrated to the product entry process and that a lot of people will need that I wouldn't consider it a bad thing for it to be core. In fact, it makes sense to be able to simply store a sku_pattern in the commerce_product_type table. If it's acceptable as a core feature, perhaps the question can become to what extent the default Product UI module allows people to configure SKU patterns or which Tokens are acceptable.

All that said, if this needs to be spun off into a custom module, I'm sure we have the necessary hooks to make it happen.