Trigger uninstall + install of eggs installed with zc.recipe.egg:custom
or :develop when pinned versions of setup-eggs have changed. To achieve
this the versions of setup-eggs are included in the section: this makes
them part of its signature so that when they change, buildout will call
`uninstall` and `install` for this section instead of just `update`.

Unlike other zc.recipe.egg entry points, :custom stores the path of the
installed egg; thus `uninstall` will remove it fully, leaving `install`
to reinstall it cleanly from scrach.

In the case of :develop, `uninstall` matters little as only the path of
the installed `.egg-link` is stored. Instead `install` must be fixed to
actually rebuild the egg in-place in the source directory and `develop`
should do nothing.

The main issue lies in `zc.buildout.easy_install.develop`: depending on
the build process, it may leave build artifacts in the source directory
that cause future runs to do nothing.