Add a "legacy" namespace and move all existing packages there. Allow for a transition period where tooling will add "legacy" to instances where no namespace is

Add a "legacy" namespace and move all existing packages there. Allow for a transition period where tooling will add "legacy" to instances where no namespace is given. Add a mechanism for legacy packages to indicate their new namespace so that transitioning could be mostly automatic for package users. Not effortless, but not necessarily very costly either.