Complete reimplementation

- Work entirely in-place, even to switch to a different version of Python.
- Update bin/buildout to immediately use the wanted Python on subsequent
  buildout runs.

For SlapOS, the second point is required to have the instanciation done
with the built Python.

slapos/rebootstrap/bootstrap.py

+import os, sys
+
+class FakeSysExecutable(object):
+
+  def __init__(self, python):
+    self.executable = python
+
+  def __getattr__(self, attr):
+    return getattr(sys, attr)
+
+def get_distributions():
+  from pkg_resources import get_distribution
+  distributions = ['setuptools', 'zc.buildout']
+  try:
+    import slapos.libnetworkcache
+  except ImportError:
+    pass
+  else:
+    distributions.append('slapos.libnetworkcache')
+  return map(get_distribution, distributions)
+
+def setup_script(path, python=sys.executable):
+  from zc.buildout import easy_install
+  try:
+    if sys.executable != python:
+      easy_install.sys = FakeSysExecutable(python)
+    easy_install.scripts(
+      ((os.path.realpath(path), 'zc.buildout.buildout', 'main'),),
+      get_distributions(),
+      python)
+  finally:
+    easy_install.sys = sys
+
+def main():
+  import shutil, subprocess, tempfile, zipfile
+  eggs_dir = sys.argv[2]
+  cache = sys.argv[3]
+  # Install setuptools.
+  src = os.path.join(cache, sys.argv[4].replace('==', '-') + '.zip')
+  tmp = tempfile.mkdtemp()
+  try:
+    with zipfile.ZipFile(src) as zip_file:
+      zip_file.extractall(tmp)
+    src, = os.listdir(tmp)
+    subprocess.check_call((sys.executable, 'setup.py', '-q', 'bdist_egg',
+                          '--dist-dir', tmp), cwd=os.path.join(tmp, src))
+    egg = os.listdir(tmp)
+    egg.remove(src)
+    egg, = egg
+    dst = os.path.join(eggs_dir, egg)
+    os.path.exists(dst) or shutil.move(os.path.join(tmp, egg), dst)
+  finally:
+    shutil.rmtree(tmp)
+  sys.path.insert(0, dst)
+  # Install other requirements given on command line.
+  from pkg_resources import working_set, require
+  from setuptools.command import easy_install
+  reqs = sys.argv[5:]
+  easy_install.main(['-mZqNxd', eggs_dir, '-f', cache] + reqs)
+  working_set.add_entry(eggs_dir)
+  for req in reqs:
+    require(req)
+  # Generate bin/buildout-rebootstrap script.
+  setup_script(sys.argv[1])
+
+if __name__ == '__main__':
+  sys.exit(main())