[ruby/rdoc] Group code object files into the same directory

(https://github.com/ruby/rdoc/pull/1114)

It's hard to distinguish code object classes by their file names alone.
And given that we have 18 such classes, it'd make the codebase a lot
easier to understand if we grouped them into a single directory.

Given that these classes are all autoloaded in `lib/rdoc.rb` instead
of required individually, this change should have minimum impact on
projects using RDoc as they generally just require `rdoc`, not individual
files. An example is Rails' `sdoc`:
https://github.com/rails/sdoc/blob/main/lib/sdoc/rdoc_monkey_patches.rb

4211292ffe

lib/rdoc/code_object/meta_method.rb

@@ -0,0 +1,6 @@
+# frozen_string_literal: true
+##
+# MetaMethod represents a meta-programmed method
+
+class RDoc::MetaMethod < RDoc::AnyMethod
+end