I was kind of tired today, so I didn't get much done. I retested after rebasing, and there was some breakage, around more detailed error messages, and some stuff that I'm sure used to work with pip, but was never supposed to, now not working. It was all test-side, though, which is nice.
I also started working on the news fragment for the change. It looks like that doesn't have to be too extravagant, so I'll focus on the documentation now.
I am... not sure how best to add this in. The solidest reasoning for this feature is constraining to pip freeze output, but that isn't made easy in all cases by this change, just possible in some instances where it currently isn't.
I'm going to need to think about this some. At least overnight.