Don't create namespaces when importing project using GH import Rake task

Stop creating namespaces that don’t exist when importing project using
GitHub import Rake task.
The user needs to provide an existing namespace which can be their own
user namespace or an existing group that they have permission to import
a project into. If the condition don't match, we return an error to the
user before creating or importing anything.

Details: https://gitlab.com/gitlab-org/gitlab/-/issues/390690

Changelog: changed