element: Handle overlaps with a whitelist and option to raise errors

* Adds the 'overlap-whitelist' field to elements' public data. This is a
  list of globs that match files that the element is allowed to overlap
  other elements with.
* Adds the project-wide 'fail-on-overlaps' field. If set,
  non-whitelisted overlaps will raise an error instead of printing a
  warning.