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, 2 deletions

diff --git a/compiler/main/GHC.hs b/compiler/main/GHC.hs
index 0e20e21fdb..825335b30a 100644
--- a/compiler/main/GHC.hs
+++ b/compiler/main/GHC.hs
@@ -291,7 +291,6 @@ import GhcPrelude hiding (init)
 import ByteCodeTypes
 import InteractiveEval
 import InteractiveEvalTypes
-import TcRnDriver       ( runTcInteractive )
 import GHCi
 import GHCi.RemoteTypes
 
@@ -358,7 +357,6 @@ import Data.Set (Set)
 import qualified Data.Sequence as Seq
 import System.Directory ( doesFileExist )
 import Data.Maybe
-import Data.List        ( find )
 import Data.Time
 import Data.Typeable    ( Typeable )
 import Data.Word        ( Word8 )