[Distutils] Update to my skeletal PEP for a new build system interface

[Robert Collins]

So I think the big thing to reconcile is the thing where I say out of
scope - using static metadata in distributions. The
dynamic/non-dynamic split may or may not be needed; but examining
source trees and sdists as different only makes sense to me if you're
gaining something - and pregenerated declarative dependencies might be
one of those things. OTOH since we haven't solved the numpy ABI
problem yet... perhaps thats premature.

btw - note that this:
----
2) There currently exist packages which do not declare consistent
metadata (e.g. ``egg_info`` and ``bdist_wheel`` might get different
``install_requires=``). When upgrading to the new system, projects
will have to evaluate whether this applies to them, and if so they
will need to stop doing that.
----

is actually a feature today. egg_info gets you the
follow-these-rules-locally with older pip versions like 1.5.4, and
bdist_wheel gets you a marker-ready set of dependencies.

-Rob