py2exe doesn't know how to handle namespace packages *at all*, so it treats
them like normal packages. As a result, if we try and bundle hgext3rd.evolve
in a py2exe build, it won't work if we install evolve into the virtualenv. In
order to work around this, tortoisehg installs hgext3rd.evolve etc into its
staged hg directory, since it doesn't use a virtualenv. As a workaround for us,
we'll just allow any extra packages users want bundled are part of hg during
the pseudo-install phase that py2exe uses. I'm not happy about this, but it
*works*.

As a sample of how you'd make an MSI with evolve bundled:

import os
import shutil
import subprocess
import tempfile

def stage_evolve(version):
  """Stage evolve for inclusion in py2exe binary."""
  with tempfile.TemporaryDirectory() as temp:
    evolve = os.path.join(temp, "evolve")
    subprocess.check_call([
        "hg.exe",
        "clone",
        "https://www.mercurial-scm.org/repo/evolve/",
        "--update",
        version,
        evolve,
    ])
    dest = os.path.join('..', 'hgext3rd', 'evolve')
    if os.path.exists(dest):
        shutil.rmtree(dest)
    shutil.copytree(os.path.join(evolve, "hgext3rd", "evolve"), dest)


def main():
  stage_evolve('tip')
  print("\0")
  print("hgext3rd")
  print("hgext3rd.evolve")
  print("hgext3rd.evolve.hack")
  print("hgext3rd.evolve.thirdparty")


if __name__ == "__main__":
  main()

is a script you can pass to the wix/build.py as --extra-packages-script,
and the resulting .msi will have an hg binary with evolve baked in. users
will still need to enable evolve in their hgrc, so you'd probably also
want to bundle configs in your msi for an enterprise environment, but that's
already easy to do with the support for extra features and wxs files in the
wix build process.