[FIX] mrp: produce SN product with lot of unbuilt product

To reproduce the issue:
1. Produce a SN product P with lot L
2. Unbuild it
3. Produce 1xP with lot L

Error: When setting the lot, an error is displayed. When marking the MO
as done, a second error is displayed

Both domains in `_onchange_lot_producing` and in the beginning of
`_check_sn_uniqueness` are incorrect. We should centralize the
uniqueness checking thanks to all existing computations in
`_check_sn_uniqueness`. This way, unbuilt/scrapped moves will be
considered

OPW-2721205

closes odoo/odoo#83345

Signed-off-by: Tiffany Chang <tic@odoo.com>