net: refactor net module to module.exports

Refactor net module to use the more efficient
module.exports = {} pattern.
Also renames internal "connect" function to "internalConnect"
to avoid collision with exported "connect".

PR-URL: https://github.com/nodejs/node/pull/11698


Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>