Refactor help scope computation logic.
Review Request #3264 - Created Dec. 16, 2015 and submitted
This is in preparation for reusing that logic for generating
the HTML options help for the site.
There's very little new code, it's mostly moving and renaming
Also moves some tests from option/ to help/, where they should
always have belonged.
Also tweaks how scope descriptions are fetched, to simplify
a method signature.
CI passes: https://travis-ci.org/pantsbuild/pants/builds/97128037