Fix custom package-db stack for non Cabal projects

Language/Haskell/GhcMod/CustomPackageDb.hs

@@ -0,0 +1,25 @@
+module Language.Haskell.GhcMod.CustomPackageDb where
+
+
+import Control.Applicative
+import Control.Monad
+import Control.Category ((.))
+import Data.Maybe
+import Data.Traversable
+import Language.Haskell.GhcMod.Types
+import Language.Haskell.GhcMod.Monad.Types
+import Language.Haskell.GhcMod.PathsAndFiles
+import Prelude hiding ((.))
+
+
+parseCustomPackageDb :: String -> [GhcPkgDb]
+parseCustomPackageDb src = map parsePkgDb $ filter (not . null) $ lines src
+ where
+   parsePkgDb "global" = GlobalDb
+   parsePkgDb "user" = UserDb
+   parsePkgDb s = PackageDb s
+
+getCustomPkgDbStack :: (IOish m, GmEnv m) => m (Maybe [GhcPkgDb])
+getCustomPkgDbStack = do
+    mCusPkgDbFile <- liftIO . (traverse readFile <=< findCustomPackageDbFile) . cradleRootDir =<< cradle
+    return $ parseCustomPackageDb <$> mCusPkgDbFile