Orleans source generator ignores implicitly referenced projects

Hello.

I've stumbled into this while trying to implement a PoC project with Orleans.
Orleans source generator (on .NET 7) will ignore grains placed in the implicitly referenced projects and will throw "unable to find implementation for.." exception.

For example, this project configuration will fail.

```mermaid
graph TD;

Silo-->Domain-->Grains
Client-->Domain
Client-->Grains
```

My suggestion is to add special assembly attribute describing which assemblies we should include in code generation.