Maps with 'value class' keys that have primitive values should be converted to indexed types

`PrototypeKey` is a primitive, so it's a valid value for an indexed type key.

```typescript
export interface Dictionary {
  protos: Map<PrototypeKey, PrototypeName>; // actual
  protos: { [key: PrototypeKey]: { [key: string]: PrototypeKey } }; // expected
}

export type PrototypeKey = string;
```