[Misc #21688] Teach RDoc about the toplevel module Ruby

Re-open the exising module by calling `rb_define_module`.
RDoc (`RDoc::Parser::C#do_classes_and_modules`) does not recognize
`rb_path2class` as a class/module definition.

Author: nobu

box.c

@@ -1057,7 +1057,7 @@ Init_Box(void)
     tmp_dir = system_tmpdir();
     tmp_dir_has_dirsep = (strcmp(tmp_dir + (strlen(tmp_dir) - strlen(DIRSEP)), DIRSEP) == 0);
 
-    VALUE mRuby = rb_path2class("Ruby");
+    VALUE mRuby = rb_define_module("Ruby");
 
     rb_cBox = rb_define_class_under(mRuby, "Box", rb_cModule);
     rb_define_method(rb_cBox, "initialize", box_initialize, 0);

version.c

@@ -132,7 +132,7 @@ Init_version(void)
      * The constants defined here are aliased in the toplevel with
      * +RUBY_+ prefix.
      */
-    VALUE mRuby = rb_path2class("Ruby");
+    VALUE mRuby = rb_define_module("Ruby");
 
     enum {ruby_patchlevel = RUBY_PATCHLEVEL};
     VALUE version = MKSTR(version);