Fix unused-import warningswip/T13064

This patch fixes a fairly long-standing bug (dating back to 2015) in
RdrName.bestImport, namely

   commit 9376249b6b78610db055a10d05f6592d6bbbea2f
   Author: Simon Peyton Jones <simonpj@microsoft.com>
   Date:   Wed Oct 28 17:16:55 2015 +0000

   Fix unused-import stuff in a better way

In that patch got the sense of the comparison back to front, and
thereby failed to implement the unused-import rules described in
  Note [Choosing the best import declaration] in RdrName

This led to Trac #13064 and #15393

Fixing this bug revealed a bunch of unused imports in libraries;
the ones in the GHC repo are part of this commit.

The two important changes are

* Fix the bug in bestImport

* Modified the rules by adding (a) in
     Note [Choosing the best import declaration] in RdrName
  Reason: the previosu rules made Trac #5211 go bad again.  And
  the new rule (a) makes sense to me.

In unravalling this I also ended up doing a few other things

* Refactor RnNames.ImportDeclUsage to use a [GlobalRdrElt] for the
  things that are used, rather than [AvailInfo]. This is simpler
  and more direct.

* Rename greParentName to greParent_maybe, to follow GHC
  naming conventions

* Delete dead code RdrName.greUsedRdrName

* Slightly change the spec of the unused function
  HsImpExpr.ieLWrappedName, and use it in RnNames.findImportUsage

1 files changed, 0 insertions, 1 deletions

diff --git a/libraries/base/Data/Functor/Classes.hs b/libraries/base/Data/Functor/Classes.hs
index e44c817b64..7c9b917608 100644
--- a/libraries/base/Data/Functor/Classes.hs
+++ b/libraries/base/Data/Functor/Classes.hs
@@ -69,7 +69,6 @@ import Control.Applicative (Alternative((<|>)), Const(Const))
 import Data.Functor.Identity (Identity(Identity))
 import Data.Proxy (Proxy(Proxy))
 import Data.List.NonEmpty (NonEmpty(..))
-import Data.Monoid (mappend)
 import Data.Ord (Down(Down))
 
 import GHC.Read (expectP, list, paren)