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.
Error handling could be done better, but I'm pretty happy with it
already.
The important part is aggregating all errors instead of failing on the
first issue we encounter.
