For people watching the bug, this is ready for review.

Most of the change is adding a "layout" script (roughly the equivalent of "make install" for Windows, but with more flexibility to generate different structures, precompile stuff, etc.)

About the only CPython change is to venv. I added a new launcher variation that looks for pyvenv.cfg and runs the python.exe listed in that, so now we don't have to copy any DLLs into the venv at all (it's needed because the old one would try to LoadLibrary DLLs inside the store package, which is not allowed, but it also fixes my #1 complaint about venv which is that upgrading the base Python breaks all venvs).

There are also a few tweaks to the startup process, including extending the existing __PYVENV_LAUNCHER__ env variable from macOS to Windows as well, though it's handled in a different location. And a couple of minor tweaks to tests - issues discovered now that it's pretty easy to generate a "real" layout to run tests in.

I can break this up into a couple of PRs if that's preferred, though it doesn't really gain anything. If you're not interested in the new layout script, stop reading the PR after you finish launcher.c :)