this is in response to
https://github.com/certbot/certbot/pull/10399#issuecomment-3166305086
this PR does two things:
1. it clarifies what is meant by "build dependencies" in DESIGN.md
2. fixes our workaround for
https://github.com/python-poetry/poetry/issues/4103 which broke when we
moved most of our code under `src` directories. i kept the previous `rm
-rf ${REPO_ROOT}/*/*.egg-info` line around for `letstest` and to
hopefully add some robustness for us if we ever move our code around
again