CHW-4304 PackageSlugConflict

package slug already exists for this repository; pick a different package_name or update the existing slug

Default message: package slug already exists for this repository; pick a different package_name or update the existing slug
HTTP status: 409 (client)

Problem. A non-deleted package slug already exists for the same (repository, package_name) tuple. Cause. A slug was created earlier and is still live. Fix. Update the existing slug with PATCH /api/packages/{id}, or delete the old slug before creating a new one. HTTP 409.