Reorganize module namespace

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

test/TargetSpec.hs

@@ -1,9 +1,9 @@
 {-# LANGUAGE OverloadedStrings #-}
 module TargetSpec where
 
-import Language.Haskell.GhcMod.Target
-import Language.Haskell.GhcMod.LightGhc
-import Language.Haskell.GhcMod.Gap
+import GhcMod.Target
+import GhcMod.LightGhc
+import GhcMod.Gap
 import Test.Hspec
 
 import TestUtils