erp5_trade: fix method `Movement_zGetPackedQuantity`
The ZSQLMethod was using a wrong table movement instead of the stock table,
hence throwing errors. I guess it might come from a legacy table, or simply no one
actually tested the script.
Part 2/x of upstreaming Nexedi ERP5 patches. I hesitated to merge directly, opening the MR just to know I not am crazy, but the patch has been applied for four months on Nexedi ERP5. Tests are currently running, awaiting results and approval.