[FIX] mrp_packaging_default: multiply packagings per products manufactured

Before this patch, the amount of packagings was the same in an `mrp.production`, no matter the ordered quantity.

This was a conceptual mistake. Packaging qtys had to be multiplied, just like it happens with UoM qtys.

@moduon MT-8145

mrp_packaging_default/models/__init__.py

@@ -1,2 +1,3 @@
 from . import mrp_bom_line
+from . import mrp_production
 from . import stock_move