https: refactor to use http internals

Rather than using `http`, use `_http_client`, etc. directly.

Also moving all the exports to the bottom, in line with most of the rest
of the codebase.

PR-URL: https://github.com/nodejs/node/pull/16395


Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>