Basically you need to define the module where the sale.order.line model resides and that would be sale module. You need to add sale module as dependencies.

You need to add depends when Odoo modules which must be loaded before this one, either because this module uses features they create or because it alters resources they define.

When a module is installed, all of its dependencies are installed before it. Likewise dependencies are loaded before a module is loaded. For more manifest file