Clear packages when copying shipments

Closes #13454
(grafted from e9535472a76c0faa5c14c4a19297bcdeb3949049)

--HG--
branch : 7.0

modules/stock_package/stock.py

@@ -414,6 +414,13 @@ class PackageMixin(object):
     def packages_moves(self):
         raise NotImplementedError
 
+    @classmethod
+    def copy(cls, shipments, default=None):
+        default = default.copy() if default is not None else {}
+        default.setdefault('packages')
+        default.setdefault('root_packages')
+        return super().copy(shipments, default=default)
+
 
 class ShipmentOut(PackageMixin, object, metaclass=PoolMeta):
     __name__ = 'stock.shipment.out'