Create a core_tasks top-level dir.
Review Request #3197 - Created Dec. 1, 2015 and submitted
|jsirois, stuhood, zundel|
Put a few of the backend/core tasks in it.
Puts some of their base classes/mixins into task/ as they are
not themselves tasks, and are intended to be reusable.
Note that there are no references to these tasks outside pantsbuild,
so it's safe to move them without a deprecation cycle.
CI passes: https://travis-ci.org/pantsbuild/pants/builds/94081549