Add option `isolate-from-buildout-and-setuptools-path` to prevent
buildout from scanning the directories where zc.buildout's or its
dependencies's currently running packages are located.

In apparently intended design, `buildout bootstrap` should already
provide isolation by either installing pinned versions of zc.buildout
and dependencies in local ./eggs directory, or copying them from the
currently running egg into ./eggs if the running versions match.

However, when the currently running versions are not eggs - which is
the case when they are installed by pip in site-packages, e.g. as is
the case in a virtual environment - then buildout merely creates
`.egg-link` references to the locations of the running packages, which
is often site-packages.

The presence of .egg-link referencing site-packages can cause packages
available in site-packages to interfere when considering which eggs to
install and what path items to insert in sys.path when creating
scripts.

This is in some way a workaround for the fact that buildout only knows
how to handle eggs and not .dist-info packages such as produced by pip.

With `isolate-from-buildout-and-setuptools-path`, buildout bootstrap
will allways install a fresh version of zc.buildout & co in ./eggs