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.

See merge request nexedi/slapos!1301

See nexedi/slapos@22cc8e7e

See merge request nexedi/slapos!1251

Includes buildout, configuration, test, and resiliency support

See merge request !1298

Allow ffmpeg to get the correct libvpx path

Using "Sort Lines Ascending" from Theia or `:'<,'>!sort -f` from vim
produce this diff.

See merge request nexedi/slapos!1279

See merge request nexedi/slapos!1295

This is more coherent with other SR. At some point, we hsould get this
file from nginx installation.

See merge request !1294

depends on erp5!1695

See merge request !1290