Make scala-refactoring compatible with unit-unloading of askLoadedTyped

The new behavior of unit unloading is introduced in
https://github.com/scala/scala/pull/3158 on which this commit is strictly
dependent.

In the longer term, scala-refactoring depends on a global
"load-all-before-index" and "remove-all-after" semantics.

It would be wise  to refactor scala-refactoring to load files parcimoniously
throughout the index building process. But this is already a step in the right
direction.