+Note [Usage for sub-bndrs]
+~~~~~~~~~~~~~~~~~~~~~~~~~~
+If you have this
+ import qualified M( C( f ) )
+ intance M.C T where
+ f x = x
+then is the qualified import M.f used? Obviously yes.
+But the RdrName used in the instance decl is unqualified. In effect,
+we fill in the qualification by looking for f's whose class is M.C
+But when adding to the UsedRdrNames we must make that qualification
+explicit, otherwise we get "Redundant import of M.C".