Move EE code out of Gitlab::ObjectHierarchy

This moves all EE specific code out of Gitlab::ObjectHierarchy and into
EE::Gitlab::ObjectHierarchy.

ee/lib/ee/gitlab/object_hierarchy.rb

+# frozen_string_literal: true
+
+module EE
+  module Gitlab
+    module ObjectHierarchy
+      # rubocop: disable CodeReuse/ActiveRecord
+      def roots
+        base_and_ancestors.where(namespaces: { parent_id: nil })
+      end
+      # rubocop: enable CodeReuse/ActiveRecord
+    end
+  end
+end

lib/gitlab/object_hierarchy.rb

@@ -39,12 +39,6 @@ module Gitlab
     end
     # rubocop: enable CodeReuse/ActiveRecord
 
-    # rubocop: disable CodeReuse/ActiveRecord
-    def roots
-      base_and_ancestors.where(namespaces: { parent_id: nil })
-    end
-    # rubocop: enable CodeReuse/ActiveRecord
-
     # Returns a relation that includes the ancestors_base set of objects
     # and all their ancestors (recursively).
     #
@@ -179,3 +173,5 @@ module Gitlab
     end
   end
 end
+
+Gitlab::ObjectHierarchy.prepend(EE::Gitlab::ObjectHierarchy)