[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/normal_module.rb

@@ -0,0 +1,73 @@
+# frozen_string_literal: true
+##
+# A normal module, like NormalClass
+
+class RDoc::NormalModule < RDoc::ClassModule
+
+  def aref_prefix # :nodoc:
+    'module'
+  end
+
+  def inspect # :nodoc:
+    "#<%s:0x%x module %s includes: %p extends: %p attributes: %p methods: %p aliases: %p>" % [
+      self.class, object_id,
+      full_name, @includes, @extends, @attributes, @method_list, @aliases
+    ]
+  end
+
+  ##
+  # The definition of this module, <tt>module MyModuleName</tt>
+
+  def definition
+    "module #{full_name}"
+  end
+
+  ##
+  # This is a module, returns true
+
+  def module?
+    true
+  end
+
+  def pretty_print q # :nodoc:
+    q.group 2, "[module #{full_name}:", "]" do
+      q.breakable
+      q.text "includes:"
+      q.breakable
+      q.seplist @includes do |inc| q.pp inc end
+      q.breakable
+
+      q.breakable
+      q.text "constants:"
+      q.breakable
+      q.seplist @constants do |const| q.pp const end
+
+      q.text "attributes:"
+      q.breakable
+      q.seplist @attributes do |attr| q.pp attr end
+      q.breakable
+
+      q.text "methods:"
+      q.breakable
+      q.seplist @method_list do |meth| q.pp meth end
+      q.breakable
+
+      q.text "aliases:"
+      q.breakable
+      q.seplist @aliases do |aliaz| q.pp aliaz end
+      q.breakable
+
+      q.text "comment:"
+      q.breakable
+      q.pp comment
+    end
+  end
+
+  ##
+  # Modules don't have one, raises NoMethodError
+
+  def superclass
+    raise NoMethodError, "#{full_name} is a module"
+  end
+
+end