lib: move _stream_wrap into internals

This makes a subsequent possible deprecation easier.

PR-URL: https://github.com/nodejs/node/pull/16158


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>