We now use tools/pip_install_editable.sh which installs our packages using the
pinned versions from certbot-auto.
We also use letsencrypt-auto-source/letsencrypt-auto instead of certbot-auto in
the root to:
1. Make sure OS bootstrappers are up to date with master.
2. Copy letsencrypt-auto-source into our tree so it can be used by
tools/pip_install_editable.sh later.
