move Jekyll::Cleaner to Jekyll::Site::Cleaner

lib/jekyll/cleaner.rb

@@ -1,6 +1,7 @@
 require 'set'
 
 module Jekyll
+  class Site
     # Handles the cleanup of a site's destination before the site is built.
     class Cleaner
       def initialize(site)
@@ -69,3 +70,4 @@ module Jekyll
       end
     end
   end
+end