[FIX] mrp: create PBM SM from confirmed MO

To reproduce the issue:
1. In Settings, enable "Multi Routes"
2. Edit the warehouse:
   - Manufacture: 2 steps
3. Create three storable products P1, P2, P3
4. Create and confirm a MO for 1 x P1 with 1 x P2
   - It should create the PBM picking
5. Add a component line for P3
6. (Because the MO is locked, the user can not edit the 'to consume'
   quantity, so) Unlock the MO
7. Set the 'to consume' qty to 1
8. Save the MO

Error: P3 is not added to PBM picking. Worst: suppose the user does
the internal transfer and then checks the availability of P3, the
line will still be unreserved

Step 6, when unlocking the MO, it also triggers a save. As a result,
the SM for P3 is created with its demand defined to 0. When adding
such SM, we do several things through this method call:
https://github.com/odoo/odoo/blob/be0b61cbaf3d3b7082aca8f96dcf8a6ee7885fea/addons/mrp/models/mrp_production.py#L776


- We will adapt its procure method (here, because of 2-steps
  manufacturing, it will be MTO)
- We will confirm the new SM -> we will run a procurement for a zero
  quantity -> it will not generate any new SM

Then, when updating the SM quantity (step 7), nothing will run a new
procurement.

Moreover, this also explains why trying to reserve the SM does not
work: it's an MTO one, but it does not have any `move_orig_ids`, so
it is not possible to assign it.

Solutions:
- It should be possible to edit the 'to consume' qty of a new SM,
  even on a locked and confirmed MO
- A procurement should be executed when updating the demand of an SM
  from 0 to >0. From 16.1, a procurement will always be executed
  each time the quantity changed (see [1]). Here, we want to limit
  the impact/risk of the fix

[1] 1f4fb64a

OPW-3253204

closes odoo/odoo#135478

Signed-off-by: William Henrotin (whe) <whe@odoo.com>