Better support for configurable & grouped products
There are three types of products in Magento. Product, SimpleProduct and GroupedProduct.
SimpleProduct is most often linked to a Product (which acts as its config). SimpleProduct can be seen as a particular variant of a Product (of a particular size and/or color).
However, there might be a SimpleProduct, which is not linked to any Product and can be bought separately.
Before this commit, all products were synced separately. However, we need all SimpleProducts which do have their Product to be grouped together and synced in one record, where SimpleProducts are nested
as "type:variant".
GroupedProducts are synced accordingly, but instead of "type:variant", they are nested with "type:member".