dns: minor refactor of dns module

Move to the more efficient module.exports = {} pattern.

PR-URL: https://github.com/nodejs/node/pull/11597


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Claudio Rodriguez <cjrodr@yahoo.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>