events: remove reaches into _events internals

[apapirovski]

This strips out the harmless bits of #17324 into this standalone PR so they can actually land.

Refactoring of the 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 (this seems to be a common reason to reach into _events in user-code as well).

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines

Affected core subsystem(s)

events