[MOV] *: _prepare_purchase_order_line

Move the method `_prepare_purchase_order_line` from 'stock.rule' to
'purchase.order.line'. The purpose is to reuse it in approvals_purchase.

task-2040710

closes odoo/odoo#41046

Related: odoo/enterprise#6702
Signed-off-by: Simon Lejeune (sle) <sle@openerp.com>