Fix extern variable cimport code in cimport_from_pyx mode

In ``cimport_from_pyx`` mode every top level .pyx construct is treated as being *defined* in a virtual .pxd. This is incorrect for ``cdef extern from`` declarations, which are never *defined*, only *declared*.
As a result, incorrect code is being generated for cimporting ``cdef extern from`` variable values, which fails at compile time or runtime.
(In fact ``cdef extern from`` variables do not need any cimport code at all)

The modified test case demonstrates the problem nicely.