[ruby/yarp] Move the gem's C extension into lib/yarp/

and change the require from "yarp.so" to "yarp/yarp", which is more
aligned with existing community conventions for gems.

https://github.com/ruby/yarp/commit/64b70e2658

lib/yarp.rb

@@ -240,4 +240,5 @@ require_relative "yarp/node"
 require_relative "yarp/ripper_compat"
 require_relative "yarp/serialize"
 require_relative "yarp/pack"
-require "yarp.so"
+
+require "yarp/yarp"