[FIX] stock: prevent merging quants during import

Before this commit:
When importing `stock.quant` with two lines with the same product and
different counted quantities, only the last line is taken.

After this commit:
Each line creates a `stock.quant`, even if a similar Quant exists.
The records will be merged at a later time by `_merge_quants`.

Steps to reproduce:
1. Create a storable product
2. Go to Inventory > Operations > Inventory Adjustments
3. Create a file with two lines with the same product,
    and different counted quantities
5. Favourites > Import Records & upload the file

OPW-3340017

closes odoo/odoo#126458

X-original-commit: 45f7368222ead5df600992c66f0da56b3a7e959a
Signed-off-by: William Henrotin (whe) <whe@odoo.com>
Signed-off-by: Walravens Mathieu (wama) <wama@odoo.com>