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

default.nix

@@ -1,6 +1,6 @@
 (import (
   let
-    lock = builtins.fromJSON (builtins.readFile ./flake.lock);
+    lock = builtins.fromJSON (builtins.readFile ./flake/dev/flake.lock);
   in
   fetchTarball {
     url =