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
../../tests/test-tutorial.t