src: clean up process.dlopen()

Move some code around and clean up the DLib helper class as prep work
for a follow-up commit.

PR-URL: https://github.com/nodejs/node/pull/18934


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Matheus Marchini <matheus@sthima.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>