refactor so we dont leak define_method into public methods

lib/rspec/core/memoized_helpers.rb

@@ -466,13 +466,16 @@ def self.module_for(example_group)
         get_constant_or_yield(example_group, :LetDefinitions) do
           mod = Module.new do
             include Module.new {
-              public_class_method :define_method
               example_group.const_set(:NamedSubjectPreventSuper, self)
+              public
+                def self.define_method *args
+                  super
+                end
             }
-
-            # Expose `define_method` as a public method, so we can
-            # easily use it below.
-            public_class_method :define_method
+            public
+              def self.define_method *args
+                super
+              end
           end
 
           example_group.__send__(:include, mod)

spec/rspec/core/memoized_helpers_spec.rb

@@ -659,5 +659,12 @@ def define_and_run_group
       end
     end
   end
+
+  describe 'Module#define_method' do
+    it 'is still a private method' do
+      a_module = Module.new
+      expect { a_module.define_method(:name) { "implementation" } }.to raise_error NoMethodError
+    end
+  end
 end