I’m happy to try to make a PoC in flit - this would be about the simplest case, so it’s a good starting point to look at the question. But I’m still a bit hazy as to what exactly I should start implementing.
The proposal from last year appears to list every file in the package individually, which seems to be an extra complication for all the practical ways I can think of to implement editable install, where we arrange to add an entire package to sys.path.
As a concrete example, I expect that if I add a new submodule in my package, that’s available in an editable install without needing to re-run an install step. That breaks if each file is individually symlinked into a new directory.