`ProjectNamespace` → `Project` relation is [added](https://gitlab.com/gitlab-org/gitlab/-/issues/337099) in `Project#project_namespace` column. To avoid ambiguity in the group / project hierarchy, we should migrate this relation to `Project#namespace` column (which is currently reserved for the relation to parent `Group` / personal namespace). Steps: - migrate all functionality built on `Project#namespace` (and expecting a `Group` / personal namespace) to work with `Project#project_namespace` - drop `Project#namespace` - rename `Project#project_namespace` → `Project#namespace`