Spitballing the improvements I want made to Poetry...
For the dev-only concept, there needs to be some kind of configuration option added, and I think it needs to be read in the Installer, and it'll do something like root = root.clone() and then del root.requires[:]. I'm not sure how many ways there are to get at logic that needs to take this into account.
For the vcs exclude override concept, there are a few ways to go, but I think the lowest-friction is to remove generated "exclude" entries if there is an exact match in "include". Okay, this actually looks really painful, because the current code doesn't engage with the ignore file from the vcs directly; all it gets is a concrete list of ignored files. The easiest way is probably some kind of "no-seed" option that just messes with the beginning of find_excluded_files.
I went elsewhere and rubber-ducked the issues here in more detail, and came to the conclusion that it's probably easier to just roll my own tool. Very slightly probably. I'm not going to do that now, so I'm going to settle for making some more activity on the issues. I will, instead, try to implement the solutions I came up with and posted in the bug comments. Just, not right now. Tomorrow.
ETA: Turns out I just wasn't specifying the include paths hard enough, so that leaves just one bug to put in a PR for at some point.