Belongs to d382a3cccf, but I authored that
on another machine.
Making sure vscode uses mypy from the environment is very important now,
because some of these errors get triggered in different ways depending
on the version.
This requires a lot of restructuring. The .nix files have to be bundled
together with the python files, so they need to follow python's module
system structure.
I ran `nix-fast-build --no-link -j 2 --eval-workers 3 --flake .#checks`
and it succeeded, so I'm reasonably confident I changed everything as
required.
