feat: Refactor the product module definitions and implementation (#6866)

There are several things done in this PR, namely:

Unify the service endpoints API to always work with a model rather than allowing to pass both ID and model (eg. both type_id and type being available in the request to create).
Start using upsertWithReplace to simplify the code and fix some deassociation bugs
Apply some changes to tests to deal with the pricing breaking changes
Correctly define the model relationships (with both ID and entity fields available)
All tests for the product are passing, which should bring us back to a great baseline.

packages/core-flows/src/definition/cart/utils/prepare-line-item-data.ts

@@ -11,6 +11,10 @@ interface Input {
 export function prepareLineItemData(data: Input) {
   const { variant, unitPrice, quantity, metadata, cartId } = data
 
+  if (!variant.product) {
+    throw new Error("Variant does not have a product")
+  }
+
   const lineItem: any = {
     quantity,
     title: variant.title,