Using `perSystem` to implement per-system attributes ties all the
per-system attributes together; all the `perSystem` functions must run
to determine output attrs. By generating them separately, the generation
can be done lazily.
Using a submodule for devShell removes the need to make every option
nullable and the need to check all of them. By using nullOr submodule,
we can tell if the value has been set and have default values for
options.
This also enables enabling a devShell with no options set.
This sets the default devShell using mkDefault, letting user set default
shells override it. Previously, to set a different default shell, one
would have to not set any of the devShell.* options and then define
devShells.default, or use mkForce.
