[build_scripts] fix find_executable for windows .bat (ie, pyside2-uic.bat)

[pmolodo]

Description of Change(s)

There was a known issue with distutils.spawn.find_executable - that it fails to find .bat files (it only looks for .exe):
https://bugs.python.org/issue2200

I noticed this problem because the executable for pyside2-uic that got installed on my system is pyside2-uic.bat - presumably this is the same for many other windows users.

Unfortunately, since distutils is now deprecated, this will never be fixed, so just made a fixed version in build.py and renamed it FindExecutable.

For comparison, original code for find_executable:
https://github.com/python/cpython/blob/master/Lib/distutils/spawn.py#L91

Fixes Issue(s)

• Building with pyside2 on windows (when pyside2-uic.bat is created)

[jilliene]

Filed as internal issue #USD-6574

[c64kernal]

Thanks for this, @pmolodo -- we'd love to not support this in the long term either. What do you think of detecting whether or not we are running Python 3.x and then using shutil.which? That way maybe this PR can be a lot smaller in that case, and the downside is that this would only be fixed on Windows in Python 3.x, but maybe that's okay for your use-case? Bonus points if we can detect the version of pyside installed and warn when the error condition would be hit? Just a thought.

[pmolodo]

That could work - I am using python 3 for windows builds, so selfishly, would be fine for me.

As for checking for error condition - I'm not sure I would trust that a .bat is always created for pyside2, and not for (original) pyside. This certainly wouldn't be the first time that it happens that my computer is special. We could just always print a warning if we fail to find something on windows + python2.

Still a problem for people who want python-2 on windows, but... one which will go away, eventually...

Only other objection I could think of would be the one mentioned here, which is that this uses PATHEXT, which is that the actual function used, spawnv, doesn't support everything in PATHEXT. However... when I tested on my machine, all the "extra stuff" came after the standard ones spawnv does accept, so... it's probably not really an issue for us.

So... I'd be on board with using shutil.which...

[c64kernal]

Okay thanks @pmolodo -- maybe best not to overthink this since this is the only report we've gotten of this problem (as far as I know) so if you only need Python 3 on Windows and shutil.which is the way of the future, let's switch to that for that platform at least for now until we can characterize further problems better :)

[pmolodo]

Pushed the change to use shutil.which - sorry to take so long to get to that!