modules: move nixpkgs module to top-level modules

Make `pkgs` available to files submodules by passing _all_ module args
through. We already did this for `specialArgs`.

modules/top-level/default.nix

@@ -7,6 +7,7 @@
   imports = [
     ../.
     ./files
+    ./nixpkgs.nix
     ./output.nix
     ./readonly-renames.nix
     ./test.nix

modules/top-level/files/default.nix

@@ -15,6 +15,15 @@ let
     modules = lib.optionals (!config.isDocs) [
       ../../.
       ./submodule.nix
+      # Pass module args through to the submodule (except `name`)
+      # Wrap each arg with the correct priority
+      {
+        _module.args = lib.pipe options._module.args [
+          lib.modules.mergeAttrDefinitionsWithPrio
+          (lib.flip builtins.removeAttrs [ "name" ])
+          (lib.mapAttrs (_: { highestPrio, value }: lib.mkOverride highestPrio value))
+        ];
+      }
     ];
     description = "Nixvim configuration";
   };

modules/top-level/nixpkgs.nix

@@ -0,0 +1,16 @@
+{ defaultPkgs, lib, ... }:
+let
+  # TODO: https://github.com/nix-community/nixvim/issues/1784
+  finalPackage = defaultPkgs;
+in
+{
+  config = {
+    # We explicitly set the default override priority, so that we do not need
+    # to evaluate finalPkgs in case an override is placed on `_module.args.pkgs`.
+    # After all, to determine a definition priority, we need to evaluate `._type`,
+    # which is somewhat costly for Nixpkgs. With an explicit priority, we only
+    # evaluate the wrapper to find out that the priority is lower, and then we
+    # don't need to evaluate `finalPkgs`.
+    _module.args.pkgs = lib.mkOverride lib.modules.defaultOverridePriority finalPackage;
+  };
+}