Support simple dependencies or ordering between plugins

I have faced this problem: I have several post scan plugins, and some need to run first some later.
There is no way to ensure this for now.
Ideally we would want to have a proper, sat-like dependency resolution mechanism. But a cruder, static way based on a relative ordering (for instance using the `sort_order` plugin class attribute) would be plenty good enough.
@yashdsaraf what do you think?