object.c: optimize rb_mod_const_get for symbol

* object.c (rb_mod_const_get): Symbol must be the entire name, not
  a nested constant path, so achieve by its ID directly.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@44189 b2dd03c8-39d4-4d8f-98ff-823fe69b080e