Warning for stateful Dagger modules

With [`@BindsInstance`](https://google.github.io/dagger/api/2.11/dagger/BindsInstance.html) in Dagger 2 and private qualifiers in modules for shared internal state, Dagger modules seem to be almost always better stateless (marked abstract with only static and `@Binds` bindings).

There may be some use case for small modules to be simplified as stateful, but a warning with the option to suppress (or promote to an error, of course) would be helpful.