Support base interfaces defined in external assemblies

[alrz]

Currently, you cannot stack Roslyn source generators to operate on generated code. As a workaround I opted into running my own generators in a separate project. That is, the refit interface is generated elsewhere, however this requires the refit generator to recognize external interfaces.

Project 1 (lib):

// <auto-generated/>
interface IExternalInterface {
  [Get("/")]
  Task<string> MyApi();
}

Project 2 (host):

// this should be recognized as a refit interface
// maybe require a [RefitInterface] attribute to hint
interface IMyApi : IExternalInterface;