Name and order the model client code

Name and order the model client code

Every package-level name of the client that sends the model calls starts
with model, and its file is model.go. The client talks to the ChatGPT
backend or to the OpenAI API, so the name tells its role and not one of
the endpoints. The interface of the turn loop that the client satisfies
is TurnClient, so it does not take the name of the client. The fake
output items of the tests are in model_test.go, next to the other fakes
of the endpoint.

The declarations of the model files follow the order of the go-code
skill. The behavior does not change.

Issue: 947bddb