What's expected to happen if there are multiple files produced in $​OUTPUT_DIR? Is it an error?

Also, doesn't this pep overlap with PEP 426? Shouldn't these ideas be incorporated there? It seems that only the command metadata is missing from PEP 426.

Another thing, I've noticed there's a "develop" command in this draft. I think this is confusing, I'd expect it to be a "install" command (and that's what it is now in setuptools). It should be renamed to something more clear, like "inplace-build" and leave the installing to pip.