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.

--exclude=build
--exclude=dist
--exclude=docs/_cache
--exclude=docs/_counter
--exclude=bower_components
--exclude=node_modules
--exclude=docs/raw
--exclude=*.egg-info
--exclude=__pycache__
--exclude=__tmpfs__