One big issue in commit e129e187 is that the
initial value of buildout_and_setuptools_path can cause buildout to use a lot
of CPU at startup. The following change would help:

    def buildout_and_setuptools_path():
        r = [get_python_lib(standard_lib=1)]
        for d in pkg_resources.working_set:
            d = d.location
            if d not in r:
                r.append(d)
        return r[1:]
    buildout_and_setuptools_path = buildout_and_setuptools_path()

but initializing buildout_and_setuptools_dists also takes time to compute,
and it's only used for bootstrap.

Because the commit is not required anymore since setuptools 36, it's easier
for us to revert partially, so we can keep applying our patches.