BackendUnavailable when specifying backend-path

[pganssle]

Environment

• pip version: 19.1.1
• Python version: 3.7.3
• OS: Linux

After extensive discussion, PEP 517 was amended to add a backend-path key to allow bootstrapped backends. I modified the build-system section of pyproject.toml in setuptools to be:

[build-system]
requires = ["wheel"]
build-backend = "setuptools.build_meta"
backend-path = ["."]

But installing setuptools with pip install -t tmp/ . is still failing with BackendUnavailable. Did anyone get around to implementing this functionality after the discussion? I don't see any PR or issue tracking it, so I figured I'd start one.

[pfmoore]

The functionality was added to pep517 in pypa/pyproject-hooks#46, but hasn't yet been added to pip. I don't think there's been a release of pep517 with this change in it yet, which we'd need if we were to vendor it in pip.

I'm not going to have any time to look at this in the near future, so if someone else wants to do the pip end of things, that would be great (I don't think it'll be hard, the heavy lifting was done in pep517 - the tests will likely be harder to write than the code :-))