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By default, don't include micropython-lib/unix-ffi in the search.

If unix_ffi=True is passed to require(), then include unix-ffi and make it
take precedence over the other locations (e.g. python-stdlib).

This does two things:
 - Prevents non-unix builds from using unix-only packages.
 - Allows the unix build to optionally use a more full-featured (e.g. ffi)
   based package, even with the same name as one from e.g. stdlib.
Signed-off-by: Jim Mussared <jim.mussared@gmail.com>