Improve WinSDK search mechanism

[CristiFati]

Found this when building for #1928.

Not sure if anyone else ran into this. I must mention that I have 9 different VStudio versions installed (and each one installed its "own" WinSDK version). At some point I did some cleanup (as there were too many WinSDKs), I don't remember if I did that from Programs and Features or from a VStudio installer (in any case I didn't manually alter the paths or registry keys).

Bottom line is that I have a discrepancy between registry and disk (1^st entry returned by registry does not exist on disk) and due to (what I consider) a bug in setup.py, I end up in the following situation:

[cfati@CFATI-5510-0:e:\Work\Dev\GitHub\CristiFati\pywin32\src]> dir /b "c:\Program Files (x86)\Windows Kits\10\Include"
10.0.10150.0
10.0.10240.0
10.0.17763.0
10.0.18362.0
10.0.19041.0
10.0.22000.0

[cfati@CFATI-5510-0:e:\Work\Dev\GitHub\CristiFati\pywin32\src]> "e:\Work\Dev\VEnvs\py_pc064_03.09_test0\Scripts\python.exe" setup.py -h
Building pywin32 3.9.304.1
Windows 10 SDK version 10.0.16299.0 is preferred, but that's not installed
Installed versions are ['10.0.17134.0', '10.0.17763.0', '10.0.18362.0', '10.0.19041.0', '10.0.22000.0']
Using 10.0.17134.0
Found Windows sdk in ['C:\\Program Files (x86)\\Windows Kits\\10\\include\\10.0.17134.0\\um'] but it doesn't appear to have windows.h

It looks like you are trying to build pywin32 in an environment without
the necessary tools installed. It's much easier to grab binaries!

Please read the docstring at the top of this file, or read README.md
for more information.

Traceback (most recent call last):
  File "e:\Work\Dev\GitHub\CristiFati\pywin32\src\setup.py", line 156, in <module>
    raise RuntimeError("Can't find the Windows SDK")
RuntimeError: Can't find the Windows SDK

even if I have valid WinSDK version(s).

Besides the fix, I also extracted registry code in a different function (will be easier to maintain in case MS decides to also release 064bit WinSDK versions), and renamed some variables.

This can be worked around by setting MSSDK_* variables, but it should also work OOTB.

[mhammond]

This seems reasonable, thanks!

[kxrob]

Note: This required a conflict resolution and rebase of #1936 , which is about to remove the obsolete _fixup_sdk_dirs() mechanism at all, including the find_platform_sdk_dir() and its changes here and the raising of "Can't find the Windows SDK". See also #1897 . Then the distutils mechanism alone will locate the VC & SDK dirs - which searches and uses the "probably best" vcvarsall.bat -> vcvars64.bat etc. (which then uses the up-to-date MS standards) compatible with python in the normative way.

(This should clear up conflicts and mixups of 2 obviously different mechanisms. For the last months at least include & lib dirs of 2 different SDK kits at the same time were seen on compiler command lines on github CI - which resulted from _fixup_sdk_dirs() )

Maybe this removes the initial problem here anyway.
Yet in case the motive for the changes here remains after that, changes / improvements may go into distutils._msvccompiler.