Support for build-and-run-time dependencies

[jdemeyer (Jeroen Demeyer)]

@uranusjr

pip can reuse the same built copy during dependant build and install, so there wouldn’t be a potential ABI mismatch during build and run time.

If you replace “pip can” by “pip must”, then I agree 100%. If the reusing is optional, then there is no difference with the status-quo.