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.

wrappers/darwin.nix

@@ -23,7 +23,10 @@ in
       default = { };
       type = types.submoduleWith {
         shorthandOnlyDefinesConfig = true;
-        specialArgs = config.lib.nixvim.modules.specialArgsWith { darwinConfig = config; };
+        specialArgs = config.lib.nixvim.modules.specialArgsWith {
+          defaultPkgs = pkgs;
+          darwinConfig = config;
+        };
         modules = [
           ./modules/darwin.nix
           ../modules/top-level

wrappers/hm.nix

@@ -22,7 +22,10 @@ in
       default = { };
       type = types.submoduleWith {
         shorthandOnlyDefinesConfig = true;
-        specialArgs = config.lib.nixvim.modules.specialArgsWith { hmConfig = config; };
+        specialArgs = config.lib.nixvim.modules.specialArgsWith {
+          defaultPkgs = pkgs;
+          hmConfig = config;
+        };
         modules = [
           ./modules/hm.nix
           ../modules/top-level

wrappers/nixos.nix

@@ -23,7 +23,10 @@ in
       default = { };
       type = types.submoduleWith {
         shorthandOnlyDefinesConfig = true;
-        specialArgs = config.lib.nixvim.modules.specialArgsWith { nixosConfig = config; };
+        specialArgs = config.lib.nixvim.modules.specialArgsWith {
+          defaultPkgs = pkgs;
+          nixosConfig = config;
+        };
         modules = [
           ./modules/nixos.nix
           ../modules/top-level

wrappers/standalone.nix

@@ -19,7 +19,9 @@ let
           mod
           ./modules/standalone.nix
         ];
-        inherit extraSpecialArgs;
+        extraSpecialArgs = {
+          defaultPkgs = pkgs;
+        } // extraSpecialArgs;
       };
       inherit (evaledModule.config) enableMan finalPackage printInitPackage;
     in