Arrange that when seeking instance decls in GHCi, in response
to a :info command, we only print ones whose types are in scope
unqualified.  This eliminates an alarmingly long list when
simply typing ':info Show', say.

On the way, I reorganised a bit.  GHCi printing happens by
converting a TyThing to an IfaceDecl, and printing that.
I now arrange to generate unqualifed IfaceExtNames directly
during this conversion, based on what is in scope.  Previously
it was done during the pretty-printing part via the UserStyle.
But this is nicer.