If you say
  ghci> import Foo( T )
  ghci> data T = MkT
  ghci> data T = XXX
then the second 'data T' should shadow the first.  But the qualified
Foo.T should still be available.  We really weren't handling this
correctly at all, resulting in Trac #8639 and #8628 among others

This patch:

* Add RdrName.extendGlobalRdrEnv, which does shadowing properly

* Change HscTypes.icExtendGblRdrEnv (was badly-named icPlusGblRdrEnv)
  to use the new function

* Change RnNames.extendGobalRdrEnvRn to use the new function

* Move gresFrom Avails into RdrName
* Better pprGlobalRdrEnv function in RdrName