SI-6989 privateWithin is now populated in reflect

Runtime reflection in JavaMirrors previously forgot to fill in
privateWithin when importing Java reflection artifacts. Now this is fixed.
introduces an exhaustive java-to-scala test

Originally composed to accommodate pull request feedback, this test has
uncovered a handful of bugs in FromJavaClassCompleter, namely:
* SI-7071 non-public ctors get lost
* SI-7072 inner classes are read incorrectly

I'm leaving the incorrect results of FromJavaClassCompleters in the check
file, so that we get notified when something changes there.
Merge pull request #2035 from scalamacros/ticket/6989

SI-6989 privateWithin is now populated in reflect