sysconfig values for `platstdlib` do not appear to be accurate

[ncoghlan]

Bug report

Bug description:

The values set for platstdlib in the sysconfig scheme paths are not consistent with the way sys.path is configured in getpath.py

sysconfig: https://github.com/python/cpython/blob/main/Lib/sysconfig/__init__.py#L28
getpath: https://github.com/python/cpython/blob/main/Modules/getpath.py#L182

Given this appears to have been wrong essentially forever, it appears nobody is using platstdlib for anything important (even I only noticed because running -m sysconfig inside a venv reports a platstdlib path inside the venv, which is definitely not right).

Still, it should be possible to make them at least a bit more consistent with the way sys.path is actually initialised (appending lib-dynload to the stdlib path looks to be sufficient for non-Windows system, but I'm less sure on Windows)

CPython versions tested on:

3.12

Operating systems tested on:

Linux, Windows

[gpshead]

Given this appears to have been wrong essentially forever, it appears nobody is using platstdlib for anything important

IMNSHO, to me that suggests we should just hunt for uses, see if they're actually needed or even correct, and whittle this down and remove the platstdlib "data". Sure, we'd follow the deprecation policy while doing so, but if the values have already been wrong forever... that'd just involve keeping wrong things in place for name/dict key lookups sake until the deprecation window closes when we just remove it.

[ncoghlan]

I started a Discourse thread with that question here: https://discuss.python.org/t/use-cases-for-the-platstdlib-path-in-sysconfig/60365

(I also posted that link to the PyPA Discord server)