Package organization: where to put 'common' modules?

[fortepianissimo]

Say I have the following package organization in a system I'm
developing:

A
 |----B
 |----C
        |----D

I have a module, say 'foo', that both package D and B require. What is
the best practice in terms of creating a 'common' package that hosts
'foo'? I want to be able to

- Testing modules in D right in D's directory and testing modules in B
in B's directory;
- If possible, minimize the modification to PYTHONPATH or sys.path
while doing the above.

Is it possible?

Thanks!