Fix #242, ghcPkgList(Ex) on NixOS

Language/Haskell/GhcMod/Monad.hs-boot

@@ -0,0 +1,16 @@
+{-# LANGUAGE RoleAnnotations #-}
+module Language.Haskell.GhcMod.Monad where
+
+import DynFlags (HasDynFlags)
+import Control.Monad.IO.Class (MonadIO)
+import Control.Applicative (Applicative)
+
+data GhcMod a
+type role GhcMod nominal
+
+instance Functor GhcMod
+instance Applicative GhcMod
+instance Monad GhcMod
+
+instance HasDynFlags GhcMod
+instance MonadIO GhcMod