Fix the module import story to match what the Revised
Haskell Report says

1. 	Don't import qualified names of things that aren't imported

2.	Fix a bug that meant
		import A hiding( D )
	where D is a data constructor, didn't work.
	[The fix is to use IEVar not IEThingAbs in the
	want_hiding case of get_item in RnNames.filterImports