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
Allows using the `callPackage(s)` pattern on tests.
Rather than using `pkgs.callPackage`, we implement our own variant using
`lib.callPackageWith`.
Our variant (`callTest`) includes additional nixvim-specific stuff
commonly used by our tests.
