Fix sol#406 by nubbing module list generated from source files

Inserting a `nub` here is strictly improving the state of affairs,
since cabal chokes on a module list with duplicates.

This patch might also fix sol#353.

src/Hpack/Module.hs

@@ -46,7 +46,7 @@ getModules dir src_ = sortModules <$> do
   if exists
     then do
       src <- Directory.canonicalizePath (dir </> src_)
-      removeSetup src . map toModule <$> getModuleFilesRecursive src
+      removeSetup src . nub . map toModule <$> getModuleFilesRecursive src
     else return []
   where
     removeSetup :: FilePath -> [Module] -> [Module]