Fix doc command not using right ghc-pkg exe

Language/Haskell/GhcMod/GhcPkg.hs

@@ -6,6 +6,7 @@ module Language.Haskell.GhcMod.GhcPkg (
   , ghcDbOpt
   , getPackageDbStack
   , getPackageCachePaths
+  , getGhcPkgProgram
   ) where
 
 import Config (cProjectVersion, cTargetPlatformString, cProjectVersionInt)
@@ -59,6 +60,18 @@ ghcDbOpt (PackageDb pkgDb)
 
 ----------------------------------------------------------------
 
+getGhcPkgProgram :: IOish m => GhcModT m FilePath
+getGhcPkgProgram = do
+  crdl <- cradle
+  progs <- optPrograms <$> options
+  case cradleProjectType crdl of
+    StackProject -> do
+        Just ghcPkg <- getStackGhcPkgPath (cradleRootDir crdl)
+        return ghcPkg
+    _ ->
+        return $ ghcPkgProgram progs
+
+
 getPackageDbStack :: IOish m => GhcModT m [GhcPkgDb]
 getPackageDbStack = do
   crdl <- cradle