Refactor __init__.py to many modules; use exthelper

This builds on !94 (merged) to do more of the same. Rather than have __init__.py as a general grab-bag of stuff, it moves everything in it into dedicated modules. All that's left is documentation and setting up the exthelper. I've added a final commit that just moves everything into hgext3rd, so we're a well-behaved citizen in the community 🙂 (@gracinet: that last part will be in a follow-up MR).

As an unfortunate side effect, using exthelper loses our docstring to hg tag, so I moved the text to the main help.