Move resources() and prep_command() out of backend/core.
Review Request #3235 - Created Dec. 11, 2015 and submitted
doc.py is the last target type in backend/core. I'll move that
in a future change, but to accommodate it now I created a new
target just for it, to isolate deps on it from deps on the
other stuff in backend/core/targets.
CI passed prior to rebasing on top of https://rbcommons.com/s/twitter/r/3228/. That rebase was a bit disruptive, so CI waiting to be run again here: https://travis-ci.org/pantsbuild/pants/builds/96331143