builtinimport.c: Recognize "namespace package" and error out as unsupported.

See http://www.python.org/dev/peps/pep-0420/#specification for spec. See
https://github.com/micropython/micropython/issues/298 for the discussion
of the implemented behavior.
builtinimport: Get the basic (and only basic) package imports work.
Propagate scope flags refactor to emitcpy.c.