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.

lib/modules.nix

@@ -1,19 +1,17 @@
 {
-  pkgs,
   lib,
-  helpers,
+  self,
 }:
 rec {
-  # Minimal specialArgs required to evaluate nixvim modules
-  specialArgs = specialArgsWith { };
-
   # Build specialArgs for evaluating nixvim modules
   specialArgsWith =
-    extraSpecialArgs:
+    # TODO: switch defaultPkgs -> pkgsPath (i.e. pkgs.path or inputs.nixvim)
+    # FIXME: Ideally, we should not require callers to pass in _anything_ specific
+    { defaultPkgs, ... }@extraSpecialArgs:
     {
+      inherit lib defaultPkgs;
       # TODO: deprecate `helpers`
-      inherit lib helpers;
-      defaultPkgs = pkgs;
+      helpers = self;
     }
     // extraSpecialArgs;