Suggestion: Improve error message when Provider/Lazy are accidentally used in multibindings

[ZacSweers]

This snippet may be accidentally written to declare a multibinding

@Module
interface Example {
  @Multibinding
  Map<String, Provider<String>> someBindings();
}

The error is that it's using a Provider in the declaration when it should just be Map<String, String>. However, the error message is a little confusing as it seems to imply that it's not a Map at all.

error: @Multibinds methods must return Map<K, V> or Set<T>

I think it would be easier for folks to debug if this message could be more explicit, such as mentioning that dagger intrinsic types like Provider and Lazy are not supported and mentioning if it sees one in the signature.

[ZacSweers]

Thanks!