Exposing APIs required by `firebase-tools` as non-internal

## Rationale

1) From https://github.com/firebase/firebase-tools/issues/1480#issuecomment-509690208

> That's what we get for relying on internal APIs

2) https://github.com/firebase/firebase-functions/issues/529

## Overview

It seems that issues caused by the necessity to expose internal APIs and usage of these APIs in `firebase-tools` starts to hurt. We should find a way to expose necessary information in a better way, that would:
- allow us to get rid of `stripInternal`, `@internal` and `@hidden` by making methods and properties actually not exposed (`private`, not `export`ed, ...);
- raise awareness of these APIs being used by other repositories, to make it more understandable that someone relies on it, to force us to think of changes to these APIs as breaking.

I'm opening this issue as a place where we can discuss possible designs and consequences, I'd be willing to work on the implementation if we decide on a certain design.

@thechenky @samtstern If you think it's a good idea, could you ping all of the people who should be involved?