Allow exposing accessors for graph extensions directly without creators

```kotlin
@DependencyGraph(isExtendable = true)
interface AppGraph {
  val loggedInGraph: LoggedInGraph
}

@ContributesGraphExtension(LoggedInScope::class)
interface LoggedInGraph {
  // ...
}
```

Main thing to keep in mind is we need to scope the instance we create if one is defined on the contributed graph (matches the Dagger behavior)