Restrict package index search_path to buildout:extra-paths value.
Without this, setting an empty extra-paths has been observed to
cause package index lookup to crash when finding an already
installed distribution in sys.path that matches the request, but
is not installable (as it is already installed, e.g. it's not a
.tar.gz or .whl or some such expected distribution format)

Maybe package index search_path should simply always be empty?
It does not seem to make much sense to look for installable
distributions on sys.path, in fact it seems accidental.