Add typedef in DependencyManager.

This is a bug fix, before this PR, Scala compiler will infer `Nothing`, which makes code below failed to compile:
```
class UserCompiler extends TransformManager(Seq(Dependency(UserPass))) {
  override def optionalPrerequisiteOf: Seq[TransformDependency] = Seq(
    Dependency[DedupModules]
  )
}
```