Windows dependency script updates

This is Good Enough to setup a system to run tests, with a little handholding. There are a few things to do, but we might as well get this in under the wire. The known issues left to address include:

• Moving the mingw-get installer to m-s.o, since the certificate on the current host expired, and the download fails by default
• Updating to the latest pyoxidizer (probably)
• Updating rustup (maybe)
• Fixing the hg wheel installed in the boostrap venv (unlikely any time soon)
• Update to modern Visual Studio (but I have no idea how to figure out the URLs/channels/etc)

The problem with the wheel is it doesn't include python3.dll and python3X.dll, so the hg.exe is bundles fails on launch. For some reason, it's not a problem when installed in the main python area, but the DLLs don't come along for the ride when setting up a venv. I worked around it for now by copying the DLLs from the main python install. The previous attempt in code was 67d14d4e036c, which was reverted by 2ec5fbe26659. d03e7e392959 was an attempt to revert that, but I never figured out why CI was flaking (it didn't happen for me locally), and now those CI systems are gone. No idea if it'll reproduce on the new setup, but I definitely won't get to it this cycle.