This module adds a constraint on stock.move.line to make sure that no stock exists for the specified
serial 1) when serial is input (in lot_name field), and 2) when the lot is being created (in _action_done())
in purchase receipts, to avoid duplicates.

Standard behavior fails to prevent the duplicates, for example,
when there is stock for a serial with no owner, and new stock comes in with the same serial with an owner.
This module intends to cater to this case.
