Reorganize module namespace

- Remove Language.Haskell prefix from all modules
- Move 'GHCMod.*' to 'GhcMod.Exe'
- Move 'GhcModExe' to 'GhcMod.Exe'

test/FindSpec.hs

@@ -1,7 +1,7 @@
 {-# LANGUAGE OverloadedStrings #-}
 module FindSpec where
 
-import GhcModExe.Find
+import GhcMod.Exe.Find
 import Test.Hspec
 import TestUtils