[mrp/anglo-saxon]: cost price based on production

*Work in Progress - not going to be merged in 8.0*

This patch solves two issues:

- manufactured product not in standard price should use, as cost price, the sum of the price of components
- when invoicing a product with a phantom BoM, the anglo-saxon entries should be on the content of the phantom bom and not the "finished" product (which is never really in stock)
done