Allow relative path names for cython freeze.

A minor change to let cython_freeze work with relative path names. cython_freeze currently assumes all modules are in the same directory when running. A command like

    cython_freeze file.py dir/lib1.py dir/lib2.py > main.c

will result in a main.c that tries to import modules "dir/lib1" and "dir/lib2". This only uses the filenames when creating the c-language import statements.