events: remove reaches into _events internals

Refactor lib & src code to eliminate all deep reaches into the
internal _events dictionary object, instead use available APIs
and add an extra method to EventEmitter: rawListeners.

PR-URL: https://github.com/nodejs/node/pull/17440


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Timothy Gu <timothygu99@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>