Normalize to NFC before comparison (fix #1810)

The filename gets normalized to NFD on OS X causing spurious failures.

Let's normalize both sides to NFC in case the Cabal file is in some
other normalization for any reason.

This is a slight hack.

src/Stack/Solver.hs

@@ -76,6 +76,8 @@ import qualified System.Directory            as D
 import qualified System.FilePath             as FP
 import           System.Process.Read
 
+import qualified Data.Text.Normalize         as T ( normalize )
+import           Data.Unicode.Types          ( NormalizationMode(NFC) )
 
 data ConstraintType = Constraint | Preference deriving (Eq)
 type ConstraintSpec = Map PackageName (Version, Map FlagName Bool)
@@ -560,8 +562,10 @@ cabalPackagesCheck cabalfps noPkgMsg dupErrMsg = do
     -- Just the latter check is enough to cover both the cases
 
     let packages  = zip cabalfps gpds
+        -- XXX taken from https://github.com/ppelleti/normalization-insensitive, see #1810
+        unicodeNormalize = T.unpack . T.normalize NFC . T.pack
         getNameMismatchPkg (fp, gpd)
-            | (show . gpdPackageName) gpd /= (FP.takeBaseName . toFilePath) fp
+            | (unicodeNormalize . show . gpdPackageName) gpd /= (unicodeNormalize . FP.takeBaseName . toFilePath) fp
                 = Just fp
             | otherwise = Nothing
         nameMismatchPkgs = mapMaybe getNameMismatchPkg packages