[FIX] stock_dropshipping: fix dropship use case in anglo saxon with perpetual valuation.

OPW: 684742
When using dropship+anglo-saxon+perpetual valuation, there is no journal
move for the delivery to debit outgoing inventory (since the goods don't
transit by an internal stock) but the sale does credit it so there was a
build up in the holding account that has to be moved out manually.
This was also reported in #12687.

The solution implemented is to check if the invoice line is related to
sale order lines having one of its procurement_ids with a purchase_line_id
set. If yes, it means that it is a confirmed dropship and in that case we
don't call to super (we don't create the cost of sale line).

That means that:
* If the procurement is in exception at the customer invoice time, the
behavior will be as it is currently, but it's fine as you don't know how
the procurement will be solved, and it'll be only at the beginning (once
the config is done it shouldn't go in exception anymore). People will have
to manually fix those amounts with a miscellaneous operation.
* users in anglo saxon mode should not use the 'stock interim account
(received)' on supplier invoices for dropshipped goods, but rather use the
COGS directly (sounds to me logical, and that's actually why I wouldn't go
for the solution to create the stock move entries every time even for the
dropshipped goods. That, and the fact that it would pollute the accounting
with useless moves).

This is a forward-port/cherry-pick of 7bdd4de8

addons/stock_dropshipping/__init__.py

 # -*- coding: utf-8 -*-
 # Part of Odoo. See LICENSE file for full copyright and licensing details.
+import models   # noqa

addons/stock_dropshipping/models/__init__.py

+# -*- coding: utf-8 -*-
+# Part of Odoo. See LICENSE file for full copyright and licensing details.
+
+import stock_dropshipping       # noqa

addons/stock_dropshipping/models/stock_dropshipping.py

+# coding: utf-8
+# Part of Odoo. See LICENSE file for full copyright and licensing details.
+
+from openerp import models, api
+
+class AccountInvoice(models.Model):
+    _inherit = 'account.invoice'
+
+    @api.model
+    def _anglo_saxon_sale_move_lines(self, i_line):
+        for sale_line in i_line.sale_line_ids:
+            for proc in sale_line.procurement_ids:
+                if proc.purchase_line_id:
+                    # if the invoice line is related to sale order lines having one of its
+                    # procurement_ids with a purchase_line_id set, it means that it is a
+                    # confirmed dropship and in that case we mustn't create the cost of
+                    # sale line (because the product won't enter the stock)
+                    return []
+        return super(AccountInvoice, self)._anglo_saxon_sale_move_lines(i_line)