A more unixy layout for cpython on windows

I'm proposing some layout changes for the upcoming 3.15 release

```
'nt': {
'stdlib': '{installed_base}/Lib',
'platstdlib': '{base}/Lib',
'purelib': '{base}/Lib/site-packages',
'platlib': '{base}/Lib/site-packages',
'include': '{installed_base}/Include',
'platinclude': '{installed_base}/Include',
'scripts': '{base}/Scripts',
'data': '{base}',
},
```

by adding the new scheme
```
'nt_conda': {
'stdlib': '{installed_base}/lib/{implementation_lower}{py_version_short}{abi_thread}',
'platstdlib': '{base}/lib/{implementation_lower}{py_version_short}{abi_thread}',
'purelib': '{base}/lib/{implementation_lower}/site-packages',
'platlib': '{base}/lib/{implementation_lower}/site-packages',
'include': '{installed_base}/include/{implementation_lower}',
'platinclude': '{installed_base}/include/{implementation_lower}',
'scripts': '{base}/bin', # maybe keep it Scripts for now and then add support to conda analogous to `python_site_packages_path`
'data': '{base}',
},
```
and then making `nt_conda` the default layout. This is what debian does to have `dist-packages` and `site-packages` split.