hgext3rd: move 'simple4server' as 'evolve.serveronly' We also more the server only extension into hgext3rd. We makes it a python sub-module of evolve for two reasons: * less polution in the hgext3rd namespace, * this make it possible to share the code between 'evolve' and 'evolve.serveronly' instead of duplicating it. note that we now install the extension too