tcltklib.c: rb_path2class

* ext/tk/tcltklib.c (ip_ruby_cmd_receiver_const_get): simply use
  rb_path2class() to get a class/module from its name.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@49029 b2dd03c8-39d4-4d8f-98ff-823fe69b080e