WIP: Add gitlab indexer DO NOT MERGE

This is still prototype code and requires unreleased changes in the
gitlab gem.

Add 'gitlab' support to cache_builder

This can scan GitLab groups looking for public and non-public
projects that contain cookbooks with semver tags (e.g. "v1.2.3").

For public projects, it uses the `location_type` of "uri".

For private projects, it uses the `location_type` of "gitlab".