[FIX] {purchase_,}mrp: prevent kit structure changes

When changing the structure of a kit, it lead to undesirable behaviors
on Odoo

Issue 01:
1. Create 3 products P_kit, P_compo01, P_compo02
    - Type: Storable
    - Category: PC
2. Create a bill of materials:
    - Product: P_kit
    - Type: Kit
    - Components:
        - 1 x P_compo01
3. Create a purchase order PO with 2 x P_kit
4. Confirm PO and process the receipt
5. Edit the bill of materials of P_kit:
    - Add 1 x P_compo02 in components
6. Return 1 x P_compo01
7. Go back to the PO

Error: The received quantity is 0 while it should be 1.0

When processing the return, the received quantity is recomputed, which
lead to:
https://github.com/odoo/odoo/blob/59fcb31f5a0b8136dae26b70ca0087f9b5cf3d24/addons/purchase_mrp/models/purchase_mrp.py#L29
However, since in the mean time the user added a new line in the BoM,
`_compute_kit_quantities` doesn't find any associated SM
(`bom_line_moves` is empty in [3]) and thus returns 0.

Issue 02:
(Need account_accountant. Use demo data)
1. Create a product category PC:
    - Costing Method: AVCO
    - Inventory Valuation: Automated
    - Set up the Price Difference Account
2. Create 3 products P_kit, P_compo01, P_compo02
    - Type: Storable
    - Category: PC
3. Create a bill of materials:
    - Product: P_kit
    - Type: Kit
    - Components:
        - 1 x P_compo01
4. Create a purchase order PO with 1 x P_kit
5. Confirm PO and process the receipt
6. Edit the bill of materials of P_kit:
    - Add 1 x P_compo02 in components
7. Create and Post the bill

Error: an Odoo Error is raised "ZeroDivisionError: float division by
zero"

While confirming the bill, some anglo saxo lines are generated. To do
so, the valuation of the kit is computed: [1]. In the above case, it
will lead to [2]. However, since in the mean time the user added a new
line in the BoM, `_compute_kit_quantities` doesn't find any associated
SM (`bom_line_moves` is empty in [3]) and thus returns 0. Back to [1],
the quantity is used to divide the total price -> it will raise an error
if this quantity is zero

Suggestion:
Such situations should not happen: once a product is used at least once,
it should not become a kit nor have a new structure (if it was already a
kit). Otherwise, `_compute_kit_quantities` will not correctly work since
it is not possible to take the BoM changes into consideration.

[1]
https://github.com/odoo/odoo/blob/abfe37fcea5b20f77799d9331d4d011530880669/addons/purchase_stock/models/account_invoice.py#L72-L74
[2]
https://github.com/odoo/odoo/blob/75191404788ab83645ee35b779991ea6fcdfa406/addons/purchase_mrp/models/stock_move.py#L19
[3]
https://github.com/odoo/odoo/blob/7d1af314320547ab5e37c1d97cad22992c98565b/addons/mrp/models/stock_move.py#L275-L294



OPW-2780855

closes odoo/odoo#86775

X-original-commit: 33fa43f7
Signed-off-by: William Henrotin (whe) <whe@odoo.com>
Signed-off-by: Adrien Widart <awt@odoo.com>