SI-5954 Invalidate TypeRef cache when opening package object

I noticed that the pos/5954 was tripping a println "assertion".
This stemmed from an inconsistency between
`TypeRef#{parents, baseTypeSeq}` for a package objects compiled
from source that also has a class file from a previous compilation
run.

I've elevated the println to a devWarning, and changed
`updatePosFlags`, the home of this evil symbol overwriting,
to invalidate the caches in the symbols info. Yuck.

I believe that this symptom is peculiar to package objects because
of the way that the completer for packages calls `parents` during
the Namer phase for package objects, before we switch the symbol
to represent the package-object-from-source. But it seems prudent
to defensively invalidate the caches for any symbol that finds its
way into `updatePosFlags`.