Fix #239 - handling of package objects

References to `.package` are now also inserted if the
accessed member comes from a class inherited by a package
object.
Fix to asSeenFrom for package object members

Like TypeAssigner, asSeenFrom needs to insert a package object if the
prefix is a package but the class enclosing the type is not.
Merge pull request #282 from dotty-staging/fix/i0239-package-objects

Fix #239 - handling of package objects