Fix imports for new mtl

mtl decided to un-export `Control.Monad` and `Data.Monoid`, which in
itself is fine, but of course it breaks everything.

src/PureNix/Convert.hs

@@ -5,6 +5,7 @@
 
 module PureNix.Convert (convert, ModuleInfo (..)) where
 
+import Control.Monad
 import Data.Bitraversable
 import qualified Data.Map as M
 import Data.Set (Set)

src/PureNix/Main.hs

@@ -17,6 +17,7 @@ import qualified System.Exit as Sys
 import System.FilePath ((</>))
 import qualified System.FilePath as FP
 import System.IO
+import Control.Monad
 
 defaultMain :: IO ()
 defaultMain = do