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Previously, slapos.rebootstrap switched from Python X.X to Python Y.Y
by first arranging to have source distributions for zc.buildout and its
dependencies, either already available in dowload cache, or downloading
them with Python X.X using internal APIs of zc.buildout, then launching
Python Y.Y to extract and bootstrap the installation of setuptools, and
then use setuptools.easy_install to install the other distributions,
meaning zc.buildout and any other dependency.

The versions of zc.buildout and its dependencies thus installed are
exactly the same as the one originally running with Python X.X. This
implies of course that these versions are compatible with Python X.X
and Python Y.Y - that is a hard constraint currently.

Thus instead of reinstalling them, we can actually just reuse the
already installed versions with Python Y.Y by passing the packages in
sys.path, giving us a working zc.buildout which we can use to properly
reinstall zc.buildout and its dependencies for Python Y.Y.

This allows for a much easier process, as we can merely achieve this
by calling buildout bootstrap with Python Y.Y without resorting to
using internal APIs of zc.buildout to download the source distributions
by hand, nor needing to manually bootstrap their installation by
extracting setuptools etc.

Note that the previous implementation also didn't respect :whl version
pins.