Refactor src/black/__init__.py into many files

[ambv]

It's the same code but in smaller chunks.

[ichard26]

Chatting with Cooper via IRC, they want this refactor to publicized in the CHANGELOG. I don't feel strongly, but I'm putting the idea out there. I think their rational was that this may introduce bugs and crashes so warning users (explicitly or implying) is a good idea.

[JelleZijlstra]

they want this refactor to publicized in the CHANGELOG

I agree. This change will likely break users who import black and use some functions. They shouldn't do that since it's undocumented and all, but the change has a reasonable chance of affecting users, so we should publicize it.

[JelleZijlstra]

Also, thanks @ambv for getting this done! This will make Black's code a lot easier to maintain and understand and I'm happy we're finally doing it!