Fix build on case-insensitive filesystems (#873)

GhcModExe/Modules.hs

@@ -0,0 +1,27 @@
+module GhcModExe.Modules (modules) where
+
+import Control.Arrow
+import Data.List
+import Language.Haskell.GhcMod.Convert
+import Language.Haskell.GhcMod.Types
+import Language.Haskell.GhcMod.Monad
+import Language.Haskell.GhcMod.Gap ( listVisibleModuleNames
+                                   , lookupModulePackageInAllPackages
+                                   )
+
+import qualified GHC as G
+
+----------------------------------------------------------------
+
+-- | Listing installed modules.
+modules :: (IOish m, Gm m)
+        => Bool -- ^ 'detailed', if 'True', also prints packages that modules belong to.
+        -> m String
+modules detailed = do
+  df <- runGmPkgGhc G.getSessionDynFlags
+  let mns = listVisibleModuleNames df
+      pmnss = map (first moduleNameString) $ zip mns (modulePkg df `map` mns)
+  convert' $ nub [ if detailed then pkg ++ " " ++ mn else mn
+                 | (mn, pkgs) <- pmnss, pkg <- pkgs ]
+ where
+   modulePkg df = lookupModulePackageInAllPackages df