[MERGE] product: refactoring of product variants.

addons/account/product_view.xml

addons/account_anglo_saxon/product_view.xml

addons/event_sale/event_sale_view.xml

addons/hr_expense/hr_expense.py

addons/hr_expense/hr_expense_view.xml

addons/membership/membership.py

addons/membership/membership_view.xml

addons/mrp/mrp.py

addons/mrp/mrp_demo.xml

addons/mrp/mrp_view.xml

addons/mrp/procurement.py

addons/mrp/product.py

addons/mrp/report/price.py

addons/mrp/security/ir.model.access.csv

addons/mrp/stock.py

addons/mrp/test/bom_with_service_type_product.yml

addons/mrp/test/order_demo.yml

addons/mrp/views/report_mrpbomstructure.xml

addons/mrp/wizard/change_production_qty.py

addons/mrp_byproduct/test/mrp_byproduct.yml