Merge XQuartz and X11 modules

Library/Homebrew/os/mac/xquartz.rb

 module OS
   module Mac
+    X11 = XQuartz = Module.new
+
     module XQuartz
       extend self
 
@@ -87,18 +89,6 @@ module OS
       def installed?
         !version.nil? && !prefix.nil?
       end
-    end
-
-    module X11
-      extend self
-
-      def prefix
-        MacOS::XQuartz.prefix
-      end
-
-      def installed?
-        MacOS::XQuartz.installed?
-      end
 
       # If XQuartz and/or the CLT are installed, headers will be found under
       # /opt/X11/include or /usr/X11/include. For Xcode-only systems, they are