[Cython] cross project cimport

[Stefan Behnel]

Hi,

Brian Granger wrote:
> It would be nice if mpi4py could install its MPI.pxd and mpi.pxi files
> and then we could do:
> 
> cimport mpi4py.MPI

The mpi4py project can do that in their setup.py by making the public files
package data. I don't currently know if Cython searches the standard package
directory of Python (site-packages), but if not, it would be a good idea to
enable that.

Stefan