Group code object files into the same directory (#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

lib/rdoc.rb

@@ -188,26 +188,26 @@ def self.home
   # programs: classes, modules, methods, and so on.
   autoload :CodeObject,     "#{__dir__}/rdoc/code_object"
 
-  autoload :Context,        "#{__dir__}/rdoc/context"
-  autoload :TopLevel,       "#{__dir__}/rdoc/top_level"
-
-  autoload :AnonClass,      "#{__dir__}/rdoc/anon_class"
-  autoload :ClassModule,    "#{__dir__}/rdoc/class_module"
-  autoload :NormalClass,    "#{__dir__}/rdoc/normal_class"
-  autoload :NormalModule,   "#{__dir__}/rdoc/normal_module"
-  autoload :SingleClass,    "#{__dir__}/rdoc/single_class"
-
-  autoload :Alias,          "#{__dir__}/rdoc/alias"
-  autoload :AnyMethod,      "#{__dir__}/rdoc/any_method"
-  autoload :MethodAttr,     "#{__dir__}/rdoc/method_attr"
-  autoload :GhostMethod,    "#{__dir__}/rdoc/ghost_method"
-  autoload :MetaMethod,     "#{__dir__}/rdoc/meta_method"
-  autoload :Attr,           "#{__dir__}/rdoc/attr"
-
-  autoload :Constant,       "#{__dir__}/rdoc/constant"
-  autoload :Mixin,          "#{__dir__}/rdoc/mixin"
-  autoload :Include,        "#{__dir__}/rdoc/include"
-  autoload :Extend,         "#{__dir__}/rdoc/extend"
-  autoload :Require,        "#{__dir__}/rdoc/require"
+  autoload :Context,        "#{__dir__}/rdoc/code_object/context"
+  autoload :TopLevel,       "#{__dir__}/rdoc/code_object/top_level"
+
+  autoload :AnonClass,      "#{__dir__}/rdoc/code_object/anon_class"
+  autoload :ClassModule,    "#{__dir__}/rdoc/code_object/class_module"
+  autoload :NormalClass,    "#{__dir__}/rdoc/code_object/normal_class"
+  autoload :NormalModule,   "#{__dir__}/rdoc/code_object/normal_module"
+  autoload :SingleClass,    "#{__dir__}/rdoc/code_object/single_class"
+
+  autoload :Alias,          "#{__dir__}/rdoc/code_object/alias"
+  autoload :AnyMethod,      "#{__dir__}/rdoc/code_object/any_method"
+  autoload :MethodAttr,     "#{__dir__}/rdoc/code_object/method_attr"
+  autoload :GhostMethod,    "#{__dir__}/rdoc/code_object/ghost_method"
+  autoload :MetaMethod,     "#{__dir__}/rdoc/code_object/meta_method"
+  autoload :Attr,           "#{__dir__}/rdoc/code_object/attr"
+
+  autoload :Constant,       "#{__dir__}/rdoc/code_object/constant"
+  autoload :Mixin,          "#{__dir__}/rdoc/code_object/mixin"
+  autoload :Include,        "#{__dir__}/rdoc/code_object/include"
+  autoload :Extend,         "#{__dir__}/rdoc/code_object/extend"
+  autoload :Require,        "#{__dir__}/rdoc/code_object/require"
 
 end