Phase 2 from #5407

Allow pip to build packages for distribution - pip build

Some notes in #5407 (comment)

pip now has PEP 517 support and uses build isolation by default on packages with pyproject.toml files -- which means the way pip builds is difficult to reproduce externally in a trivial manner.

The pip build command would use the same code paths as PEP 517 with the sole difference being that it does not install the packages. It's essentially the pip wheel in a PEP 517 world.