Support source in multiple files

Created originally on Bitbucket by paugier (Pierre Augier)

This should work:

# util.py

import h5py

const = 1
foo = 1

def func():
    return const

def use_h5py():
    ...

Another file:

from util import func, foo

from transonic import jit

const = 2

@jit
def main():
    print(foo, func(const))

Note the possible name clash if everything is written on one Pythran file (and there is no namespace keyword in Python!).

I think Pythran supports source in more that one file, so we could use that.