Using untyped dicts was not a safe way to pass arguments to error
message rendering. Also, having one giant match statement wouldn't scale
well. Using function pointers and kwargs, mypy can now properly check
whether all required arguments are passed to DiskoMessage().
As a bonus, it's now easy to use the "Go to definition" LSP command for
error codes, which wasn't possible before.
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.
