I think a type alias that is used only once `MultiHashMap` is probably not that helpful. I'd just inline it so there's one less thing to lookup when one is reading the code.

I think the same goes for the `Responses` alias. In particular I prefer to read:

```scala
override def default(key: SourceFile): Set[Response[Tree]] = Set()
```

than 

```scala
override def default(key: SourceFile): Responses = Set()
```