Properly handle Akeneo product "type" change

[mmenozzi]

Let's assume that on Akeneo there's a simple Product with code A and this product has been already imported to Sylius having a Sylius Product with code A and a ProductVariant with code A.

Let's also assume that on Akeneo a new Product Model with code B is created and then the existing Product A is added to it using the "Add to an existing product model" bulk action.

In such situation, the subsequent Sylius import will fail the following error:

In ORMInvalidArgumentException.php line 114:

  A new entity was found through the relationship 'App\Entity\Product\ProductVariant#product' that was not
  configured to cascade persist operations for entity: . To solve this issue: Either explicitly call Entity
  Manager#persist() on this unknown entity or configure cascade persist this association in the mapping for
   example @ManyToOne(..,cascade={"persist"}).