Don't rely on ghc-mod being in PATH

Language/Haskell/GhcMod/Find.hs

@@ -27,12 +27,13 @@ import Exception (ghandle, handleIO)
 import qualified GHC as G
 import Language.Haskell.GhcMod.Convert
 import Language.Haskell.GhcMod.Monad
+import Language.Haskell.GhcMod.Utils
 import Language.Haskell.GhcMod.Types
 import Name (getOccString)
 import System.Directory (doesDirectoryExist, getAppUserDataDirectory, doesFileExist, getModificationTime)
 import System.FilePath ((</>))
 import System.IO
-import System.Process (readProcess)
+import System.Environment (getExecutablePath)
 
 #ifndef MIN_VERSION_containers
 #define MIN_VERSION_containers(x,y,z) 1
@@ -84,9 +85,18 @@ lookupSymbol opt sym db = convert opt $ lookupSymbol' sym db
 loadSymbolDb :: IO SymbolDb
 loadSymbolDb = SymbolDb <$> readSymbolDb
 
+ghcModExecutable :: IO FilePath
+ghcModExecutable =
+#ifndef SPEC
+    getExecutablePath
+#else
+    return "dist/build/ghc-mod/ghc-mod"
+#endif
+
 readSymbolDb :: IO (Map Symbol [ModuleString])
 readSymbolDb = handle (\(SomeException _) -> return M.empty) $ do
-    file <- chop <$> readProcess "ghc-mod" ["dumpsym"] []
+    ghcMod <- ghcModExecutable
+    file <- chop <$> readProcess' ghcMod ["dumpsym"]
     M.fromAscList . map conv . lines <$> readFile file
   where
     conv :: String -> (Symbol,[ModuleString])