Fix haddocks for global packages on Windows (#375)

src/Stack/GhcPkg.hs

@@ -36,8 +36,8 @@ import           Prelude hiding (FilePath)
 import           Stack.Build.Types (StackBuildException (Couldn'tFindPkgId))
 import           Stack.Constants
 import           Stack.Types
-import           System.Directory (createDirectoryIfMissing, doesDirectoryExist, canonicalizePath)
-import           System.FilePath (normalise)
+import           System.Directory (createDirectoryIfMissing, doesDirectoryExist, canonicalizePath,
+                                   doesDirectoryExist)
 import           System.Process.Read
 
 -- | Get the global package database
@@ -137,7 +137,13 @@ findGhcPkgHaddockHtml :: (MonadIO m, MonadLogger m, MonadBaseControl IO m, Monad
 findGhcPkgHaddockHtml menv pkgDbs pkgId = do
     mpath <- findGhcPkgField menv pkgDbs (packageIdentifierText pkgId) "haddock-html"
     case mpath of
-        Just !path -> return $ parseAbsDir $ normalise $ T.unpack path
+        Just !path0 -> do
+            let path = T.unpack path0
+            exists <- liftIO $ doesDirectoryExist path
+            path' <- if exists
+                then liftIO $ canonicalizePath path
+                else return path
+            return (parseAbsDir path')
         _ -> return Nothing
 
 -- | Get the dependencies of the package.