Support describing nested messages about evans HOT 2 OPEN

$ evans -r cli desc
api.Example:
service Example {
  rpc SayHello ( .api.SayHelloRequest ) returns ( .api.SayHelloResponse );
}

$ evans -r cli desc api.SayHelloRequest
api.SayHelloRequest:
message SayHelloRequest {
  Person person = 1;
}

$ evans -r cli desc api.Person
api.Person:
message Person {
  // something
}