Introduce macro.pythonpath.egg to compute during install the PYTHONPATH
of setup eggs required to install other eggs, and insert the PYTHONPATH
into the environment section such as used by zc.recipe.egg:custom.

Both numpy and scipy use a Python subprocess to call Cython in their
setup.py, therefore the setup-eggs option of zc.recipe.egg:custom
does not work because sys.path is not propagated to the subprocess.