core: Keep refactoring, this time to prepare for generator sources. - Make a few APIs simpler. - Content pipelines create their own jobs, so that generator sources can keep aborting in `getContents`, but rely on their pipeline to generate pages for baking.

*.pyc
*.egg-info
.cache
__pycache__
venv
tags
bower_components
node_modules
build/lib
util/messages/_cache
util/messages/_counter
dist
docs/_cache
docs/_counter
foodtruck/static
piecrust.egg-info
piecrust/__version__.py
.ropeproject