Fix dropExeExtension on Windows

On Windows dropExeExtension doesn't actually drop the exe extension.

We have

    splitExtension "foo.exe" == ("foo", ".exe")

but the code is expecting just "exe" for the extension field.

Among other things this makes guessToolFromGhcPath behave
unexpectedly. Since takeVersionSuffix can't see past the extension the
`null suf` case in mkGuesses will trigger which causes the versioned
program path candidates to not be looked for.

Cabal/Distribution/Simple/Utils.hs

@@ -1415,14 +1415,14 @@ dropExeExtension filepath =
     (filepath', extension) | extension `elem` exeExtensions -> filepath'
                            | otherwise                      -> filepath
 
--- | List of possible executable file extensions on the current platform.
+-- | List of possible executable file extensions on the current build platform.
 exeExtensions :: [String]
 exeExtensions = case buildOS of
   -- Possible improvement: on Windows, read the list of extensions from the
   -- PATHEXT environment variable. By default PATHEXT is ".com; .exe; .bat;
   -- .cmd".
-  Windows -> ["", "exe"]
-  Ghcjs   -> ["", "exe"]
+  Windows -> ["", ".exe"]
+  Ghcjs   -> ["", ".exe"]
   _       -> [""]
 
 -- ------------------------------------------------------------

Cabal/tests/UnitTests/Distribution/Simple/Utils.hs

@@ -8,7 +8,8 @@ import Distribution.Verbosity
 import Data.IORef
 import System.Directory ( doesDirectoryExist, doesFileExist
                         , getTemporaryDirectory
-                        , removeDirectoryRecursive, removeFile )
+                        , removeDirectoryRecursive, removeFile
+                        , exeExtension )
 import System.IO (hClose, localeEncoding, hPutStrLn)
 import System.IO.Error
 import qualified Control.Exception as Exception
@@ -84,6 +85,10 @@ rawSystemStdInOutTextDecodingTest ghcPath
     Left err | isDoesNotExistError err -> Exception.throwIO err -- no ghc!
              | otherwise               -> return ()
 
+dropExeExtensionTest :: Assertion
+dropExeExtensionTest =
+  assertBool "dropExeExtension didn't drop exeExtension!" $
+    dropExeExtension ("foo" ++ exeExtension) == "foo"
 
 
 tests :: FilePath -> [TestTree]
@@ -98,4 +103,6 @@ tests ghcPath =
       withTempDirRemovedTest
     , testCase "rawSystemStdInOut reports text decoding errors" $
       rawSystemStdInOutTextDecodingTest ghcPath
+    , testCase "dropExeExtension drops exe extension" $
+      dropExeExtensionTest
     ]