`pip install <package>` produces a `<package-name>` package folder
and a `<package-name>.dist-info` metadata folder, which is another
format than eggs. Then buildout bundles both folders into a parent
folder `<package.egg>` and tries to act as though it were an egg.

Instead, use `pip wheel` to produce a wheel - which `pip install`
does internally anyway - and `setuptools.Wheel.install_as_egg` to
produce a genuine egg.

This is much cleaner: it consistently produces genuine eggs instead
of sometimes true eggs, sometimes `.dist-info` bundles depending on
whether `pip install` is called or the package was installed from a
`.whl` or `.egg` archive directly.

The only downside it this requires setuptools >= 38.2.3.