Generators assembly needs at least one generator deriving directly from SourceGenerator to execute

[carldebilly]

Steps to reproduce :

1. Create a project A with an abstract generator GenA deriving from SourceGenerator
2. Create a project B, referencing project A and create a generator GenB deriving deriving from GenA
3. Try to run GenB in a project, the Uno.SourceGeneratorTasks won't see any useful generators to run.
4. Add a generator GenC diriving directly from SoureGenerator into Project B.
5. Both GenB and GenC will execute correctly.