[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/single_class.rb

@@ -0,0 +1,30 @@
+# frozen_string_literal: true
+##
+# A singleton class
+
+class RDoc::SingleClass < RDoc::ClassModule
+
+  ##
+  # Adds the superclass to the included modules.
+
+  def ancestors
+    superclass ? super + [superclass] : super
+  end
+
+  def aref_prefix # :nodoc:
+    'sclass'
+  end
+
+  ##
+  # The definition of this singleton class, <tt>class << MyClassName</tt>
+
+  def definition
+    "class << #{full_name}"
+  end
+
+  def pretty_print q # :nodoc:
+    q.group 2, "[class << #{full_name}", "]" do
+      next
+    end
+  end
+end