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 !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 !1279

See merge request !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