flake: partition dev dependencies

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

flake/dev/list-plugins/default.nix

@@ -1,12 +1,8 @@
-{ inputs, self, ... }:
+{ self, ... }:
 {
   perSystem =
     {
-      self',
-      config,
       lib,
-      inputs',
-      system,
       pkgs,
       ...
     }:
@@ -33,8 +29,6 @@
             list-plugins --root-path ${self} > $out
           '';
 
-    }
-    // lib.optionalAttrs (inputs.devshell ? flakeModule) {
       devshells.default.commands = [
         {
           name = "list-plugins";