Thanks for working on this.

Overall I like the idea, but have some comments/questions

1)  *Please*, *please*, *please* let's start doing PEP conversations as PRs to pypa/interoperability-peps :  )  There's a place in there for unnumbered PEPs.  Review will be better and faster IMO as PRs.  If the PR gets too cluttered with old conversations, just start clean with a new PR.   We can announce the PRs here, but don't discuss them on the mailing list.   The same goes for trimming down PEP426 as you mention.   Let's do that as a PR to pypa/interoperability-peps to the existing document.

2) Ditto on Ralf's concerns about readability.  I only really understood it after seeing the examples you gave to Ralf (they need to be in the PEP, not just in response to Ralf).   There's a few places I'd want to alter the wording, but I likely won't bother, unless it's done as a PR.  

3) You refer to "source distribution".  What does that mean exactly in this PEP?  just the current setuptools notion?  

4) Although using a process interface is not necessarily a problem, I don't agree with your point on why a python interface would be unworkable.  You're assuming that pip would try to import all the build tools (for every dependency it's processing) in the master process.  An alternative could be that pip would have it's own tool (that runs as a subprocess in an isolated env) that knows how to load and work with python build interfaces.   You could argue that a python api is  an advantage, because build tools aren't forced to grow a certain cli for pip, they just have to add a shim module that conforms to the interface.   But my point is not to argue for the Python API, but for you to remove an argument that from what I can tell, isn't really an argument for one or the other.

5) at one pt you said "--dump-build-requires would just output a constant string: {"schema_version": "2.0", "build_requires": []}"   you meant "metadata_version", right?

6)  Can you explain better why we need to manage a pypa.yaml schema version *and* a build description schema version.   Why not just a version for pypa.yaml, and if anything changes (in the yaml schema or the build "description" api),  then just bump the version.

7) it's unclear when pip get's to run "dist-info" and when the result might be different.   For example, we've discussed that run time dependencies may get clarifed *after* the build process.... so this command might produce different results at different times?  

--Marcus