Better support for cl-labels inside namespaces.

* namespace-test.el (test-i.1, test-i.2, test-i.3, test-i.4)
(test-i.5, test-i.6): New tests.

* namespace.el (namespace--function-expander): Make some sanity
checks before rewriting.
(namespace--cl-labels-expander, namespace--macrolet-env):
cl-labels needs special handling.
(namespace--lexical-let-expander, namespace--labels-expander)
(namespace--flet-expander): Don't support these obsolete versions.