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/files.nix

@@ -6,6 +6,8 @@
   ...
 }:
 let
+  builders = lib.nixvim.builders.withPkgs pkgs;
+
   fileTypeModule =
     {
       name,
@@ -73,10 +75,10 @@ let
             then
               if lib.isDerivation config.source then
                 # Source is a derivation
-                helpers.byteCompileLuaDrv config.source
+                builders.byteCompileLuaDrv config.source
               else
                 # Source is a path or string
-                helpers.byteCompileLuaFile derivationName config.source
+                builders.byteCompileLuaFile derivationName config.source
             else
               config.source;
         };