Standardising editable mode installs (runtime layout - not hooks!)

If executable .pth are on the way out, then I’d put the optional develop hook proposal back on the table as I sketched there, with symlinks in wheel as second choice. Oops, sorry, back to the interface design.

I really think we must give freedom to back-end developers (generic or custom alike) to do what they find best for their users to make the code importable.