Interface implementation returning inner class isn't correctly patched

Interface and class in the same file. Method returns inner class.
```csharp
public interface ITest {
Test.Builder Create(Entity entity, string prefixId);
}

public class Test : ITest {
Builder ITest.Create(Entity entity, string prefixId) {
// ...
return new Builder();
}

public class Builder { }
}
```
FSR produces:
```csharp
public interface ITest__Patched_{
Test.Builder Create(Entity entity, string prefixId);
}

public class Test__Patched_: ITest { // error CS0535
Builder ITest.Create(Entity entity, string prefixId) { // error CS0539
// ...
return new Builder();
}

public class Builder { }
}
```
error CS0535: 'Test__Patched_' does not implement interface member 'ITest.Create(Entity, string)'
error CS0539: 'Test__Patched_.Create(Entity, string)' in explicit interface declaration is not found among members of the interface that can be implemented

Moving interface out of the file doesn't help. Same CS0539 and CS0535 errors.

Start by reproducing the shown C# example with an interface and an inner class whose method returns that inner class. Trace the patching of the interface and implementing class, then verify that the generated code compiles without CS0535 or CS0539, both when the interface is in the same file and when it is moved.

Vom Indexierungsmodell aus dem Issue-Text verfasst.