Reorganize module namespace

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

GhcMod/Exe/Lang.hs

@@ -0,0 +1,10 @@
+module GhcMod.Exe.Lang where
+
+import DynFlags (supportedLanguagesAndExtensions)
+import GhcMod.Convert
+import GhcMod.Monad
+
+-- | Listing language extensions.
+
+languages :: IOish m => GhcModT m String
+languages = convert' supportedLanguagesAndExtensions