install: correct sysconfig scheme lookup on Windows

[lazka]

distutils by default passes os.name, so "nt", to _resolve_scheme() which results in the layout key being "", which results in get_preferred_scheme() raising, and "nt" being returned as as a fallback.

This is not correct in case sysconfig has a different scheme configuration, or in case we are installing into a venv, where the "venv" scheme should be selected and not "nt".

Fix by defaulting to the "prefix" layout in case no layout is given. This gives us "nt" from sysconfig, and "venv" in case we are in a venv. With CPython the change from "nt" to "venv" doesn't change anything for distutils since that only means "headers" are missing, which distutils injects later, resulting in the same scheme paths.

For MINGW Python which defaults to the "posix" scheme this fixes the default
install location (up until now MINGW Python patched the "nt" scheme to work
around this but we want to fix the real issue now)

[lazka]

Since it might not be obvious, here is the reason sysconfig returns "venv" in case "prefix" is passed: https://github.com/python/cpython/blob/23b93770f6e6c6db2eef9d9b9e6ca7741489ae99/Lib/sysconfig/__init__.py#L300-L301

[MohammedAlkindi]

The premise holds, and the mechanism is narrower than the description suggests. _resolve_scheme passes key from name.partition('_'), which is the empty string for "nt" because that name has no underscore, and sysconfig.get_preferred_scheme('') raises KeyError(''). So every "nt" lookup takes the except path. With key or "prefix" it returns 'nt' outside a venv and 'venv' inside one.

Worth stating in the PR: this narrows the silent fallback rather than removing it. osx_framework_library gives key='framework_library', which still raises both before and after.

Test suite on Windows: 4 failed, 234 passed, identical failing set with and without the patch.

Measurements

Python 3.14.7, per scheme name, key taken from name.partition('_'):

name='nt'                    key=''                  before=KeyError('')                  after='nt'
name='posix_prefix'          key='prefix'            before='nt'                          after='nt'
name='nt_user'               key='user'              before='nt_user'                     after='nt_user'
name='posix_home'            key='home'              before='posix_home'                  after='posix_home'
name='osx_framework_library' key='framework_library' before=KeyError('framework_library') after=KeyError('framework_library')

Inside a venv (3.13.13): sys.prefix != sys.base_prefix is True and get_preferred_scheme('prefix') returns 'venv'.

Suite, unpatched and patched, identical set:
4 failed, 234 passed, 28 skipped, 2 xfailed
The 4 are machine-local, not this patch: two test_mingw.py cases fail FileNotFoundError [WinError 2] (no MinGW on this box) and two test_extension.py cases fail ImportError: DLL load failed ... Application Control.