Linear roots scope

We need a linear version of https://gitlab.com/gitlab-org/gitlab/blob/master/ee/lib/ee/gitlab/object_hierarchy.rb#L7. This will get us some_groups.roots.

1. Create MR of roots scope to supplement the method in https://gitlab.com/gitlab-org/gitlab/blob/master/ee/lib/ee/gitlab/object_hierarchy.rb#L7

The below points were part of the original issue but have been moved to #349090

2. Replace uses of https://gitlab.com/gitlab-org/gitlab/blob/master/ee/lib/ee/gitlab/object_hierarchy.rb#L7 with the roots scope.
3. Remove https://gitlab.com/gitlab-org/gitlab/blob/master/ee/lib/ee/gitlab/object_hierarchy.rb#L7