pip-compile with no setup.py

[JulienPalard]

What's the problem this feature will solve?

With a mostly empty setup.py, say __import__("setuptools").setup(), and all requirements in setup.cfg, pip-compile setup.py works as expected.

But this mostly empty setup.py less and less usefull, as it's allowed to not have one since setuptools 40.0.9 while using PEP517/518 builds.

But without this setup.py I think we have no way to tell pip-compile: "Generate a requirements.txt for this given project.".

Describe the solution you'd like

I think it would be great to allow something like pip-compile PATH with a path to a project, like . for the current project.

[JulienPalard]

Duplicate of #1047.