Set maximum number of packages for ShipmentPacker.pack

@costford, @csaunders 

This moves the logic from https://github.com/Shopify/shopify/pull/20563 into Active Shipping.

When `ShipmentPacker.pack` attempts to pack more than `EXCESS_PACKAGE_QUANTITY_THRESHOLD` (defaulted to 10,000) packages, it will raise with a `ExcessPackageQuantity` exception.