[FIX] stock: ensure the product policy of a storage category

When processing several SMLs at once, the 'same product' policy of a
storage category is not respected

To reproduce the issue:
1. In Settings, enable:
   - Multi-Step Routes
   - Storage Categories
   - Packages
2. Create a Storage Category SC:
   - Allow New Product: same
3. Create two locations L1, L2:
   - Parent: WH/Stock
   - Type: Internal
   - Storage Category: SC
4. Create a putaway rule:
   - When in: WH/Stock
   - Package type: Pallet
   - Store to: WH/Stock
   - Having Category: SC
5. Edit the warehouse:
   - Incoming Shipments: 2 steps
6. Create two products P01, P02:
   - Type: Storable
7. Create a planned receipt R:
   - To: WH/Input
   - Operations:
     - 1 x P01
     - 1 x P02
8. Mark R as Todo
9. Create two packages:
   - 1 x P01 in PK01 (! PK01 must be a Pallet)
   - 1 x P02 in PK02 (! PK02 must be a Pallet)
10. Validate R
11. Open the related internal transfer

Error: Both packages are redirected to L1. Considering the product
policy of SC, one line should be redirected to L1 and the second one
to L2

To apply the product policy, the code looks at the quants of each
location. But it does not consider the incoming SMLs. Therefore, when
applying the putaway rule to the first SML, it selects L1 (which
makes sense). Then, for the second SML, because it does not see the
first one, it considers that L1 is empty and can be selected, hence
the error.

OPW-3204924

closes odoo/odoo#122616

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