[5.x] Remove dependency on product model

[Jade-GG]

Using ->withOnly([]) on the product relation mostly did the trick, however this did not take care of any extra relations that may be added using global scopes. This gave me two other options to choose from:

• Make a "base" product model in the Rapidez core which can be used for things like this
• Factor out the need of using the product model at all

As it turned out, we only used the product model in this package to retrieve the SKU. This made the second option especially appealing for me in this case. In this PR, I've removed the product relation and kept it only as a join.

[indykoning]

I may be missing a bit of context, but aren't we using any product information to show the products currently in the wishlist?
Like price, thumbnail, or any custom values that may be useful for certain stores

[Jade-GG]

I may be missing a bit of context, but aren't we using any product information to show the products currently in the wishlist? Like price, thumbnail, or any custom values that may be useful for certain stores

All that data comes from ES. Any special data (like comments on individual products) come from the WishlistItem model itself. I triple checked to make sure, we don't use anything from the product model other than the SKU.