Support `internal` visibility for `@MergeComponent` components

[vRallev]

This is supported by kotlin-inject and should be supported for the @MergeComponent annotation, e.g.

@AppScope
@MergeComponent(AppScope::class)
internal abstract class AppComponent

Note that @ContributesSubcomponent interfaces must be public.

Implementation notes:

• Verify this works for the MergeComponent interface too
• Verify interfaces and abstract classes