Reorganize module namespace

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

GhcMod/Exe/Flag.hs

@@ -0,0 +1,9 @@
+module GhcMod.Exe.Flag where
+
+import qualified GhcMod.Gap as Gap
+import GhcMod.Convert
+import GhcMod.Monad
+
+-- | Listing of GHC flags, same as @ghc@\'s @--show-options@ with @ghc >= 7.10@.
+flags :: IOish m => GhcModT m String
+flags = convert' Gap.ghcCmdOptions