Support a _platformhints module to request info from the underlying platform

[ncoghlan]

(Note: this proposes a new installer/platform interface, so actually pursuing it would require a PEP creating a new interface specification under https://packaging.python.org/specifications/)

What's the problem this feature will solve?

The advice that pip gives to users in error messages isn't necessarily correct, but the correct advice ends up being platform dependent.

#5346 is one example where we were able to tweak pip's behaviour based on metadata modified by the system, but it would be preferable if there was a way for particular Python installations to at least provide platform-specific wording for warnings and error messages (or to indicate that the warnings/errors shouldn't be emitted at all)

Describe the solution you'd like

The simplest solution I can see is an optional _platformhints module with a defined API that is blocked from upload to PyPI, but specified in a PyPA specification (similar to the _manylinux module in PEP 513).

Alternative Solutions

We could potentially try to define a flat metadata file instead, but I think we're going to want to be able to pass things like package names into this, as well as potentially passing in external dependency specifications.

Additional context

(@ewdurbin @dstufft Could one of you flag this name as blocked until we've resolved this design discussion?)