Migrate flake outputs, config files, etc from buildbot-nix to nixbot.
This is not urgent, because nixbot falls back to buildbot-nix.toml when
nixbot.toml is missing.
This removes the need for end-users to manually set
`nixvim.inputs.devshell.follows = ""` (etc)
We offload evaluation of some of our flake modules into a `dev`
partition submodule.
- When its not needed, this submodule is not evaluated.
- When it is needed, it fetches extra inputs from `flake/dev/flake.nix`
as part of evaluating the submodule.
See https://flake.parts/options/flake-parts-partitions.html
Matt Sturgeon