[IMP] stock: performance on _compute_product_availability
[IMP] stock: performance on _compute_product_availability

Improve performance (*3) on _compute_product_availability
`mapped` makes use of orm prefetch, which is inefficient in this use
case when facing high volume.

closes odoo/odoo#30545
ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo