Declare public API names

[cjwatson]

mypy --strict enforces these by default.

I'm not sure whether Emitter should be public. It doesn't have an underscore, but it also isn't used by charmcraft - I guess it might be needed to implement test fixtures? I decided to err on the side of strictness and you can always open it up if you want.

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[facundobatista]

Hola Colin,

We have mixed feelings about the "publicness" of Emitter. As a name, it's not used from outside (so the PR here is fine, will land it). But it's the "main interface with all the message system", so it's weird to declare it "private".

Thanks for this work!

[facundobatista]

Thanks!