lib: segregate and deprecate functions that need pkgs

Splits everything that depends on a `pkgs` instance into an optional
attrs, allowing `helpers.nix` to be bootstrapped without `pkgs`.

This required some refactoring:
- `modules.specialArgs` is only available when `pkgs` is used
- `modules.specialArgsWith` now requires `defaultPkgs` be provided
- `builders.*` now have `*With` variants that take `pkgs` as an argument
  and a `withPkgs` function that returns the old interface
- Had to define the fixed part of `builders` outside the attrs for now,
  to avoid infinite recursion.
- The old `builders` are now deprecated, and print a warning when
  evaluated
- `withOptoinalFns` was introduced to merge the optional attrs into the
  final lib.

modules/top-level/files/submodule.nix

@@ -3,7 +3,6 @@
   config,
   lib,
   pkgs,
-  helpers,
   ...
 }:
 {
@@ -18,7 +17,8 @@
   config =
     let
       derivationName = "nvim-" + lib.replaceStrings [ "/" ] [ "-" ] name;
-      writeContent = if config.type == "lua" then helpers.writeLua else pkgs.writeText;
+      writeContent =
+        if config.type == "lua" then lib.nixvim.builders.writeLuaWith pkgs else pkgs.writeText;
     in
     {
       path = lib.mkDefault name;