modules: refactor module bootstrapping

Let's simplify things by defining all modules in `./plugins`, `./modules`
and `./wrappers/modules`.

Instead of currying `pkgs` into a bootstrapping module, we can require
`defaultPkgs` be provided as a special arg.

This refactor allows us to completely remove `flake-modules/modules.nix`!

modules/misc/default.nix

@@ -0,0 +1,13 @@
+{ defaultPkgs, ... }:
+let
+  # We can't use config._module.args to define imports,
+  # so we're forced to use specialArgs.defaultPkgs's path
+  nixosModules = defaultPkgs.path + "/nixos/modules/";
+in
+{
+  imports = [
+    ./nixpkgs.nix
+    ./nixvim-info.nix
+    (nixosModules + "/misc/meta.nix")
+  ];
+}

modules/misc/nixpkgs.nix

@@ -0,0 +1,24 @@
+{
+  defaultPkgs,
+  pkgs,
+  lib,
+  ...
+}:
+let
+  # TODO: https://github.com/nix-community/nixvim/issues/1784
+  finalPackage = defaultPkgs;
+in
+{
+  config = {
+    _module.args = {
+      # 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`.
+      pkgs = lib.mkOverride lib.modules.defaultOverridePriority finalPackage;
+      inherit (pkgs) lib;
+    };
+  };
+}

modules/misc/nixvim-info.nix

@@ -0,0 +1,48 @@
+{ lib, ... }:
+with lib;
+{
+  # Attribute may contain the following fields:
+  #  - path: Path to the module, e.g. [ "plugins" "<name>" ]
+  #  - description: A short description of the plugin
+  #  - url: Url for the plugin
+  #
+  # We need to use an attrs instead of a submodule to handle the merge.
+  options.meta.nixvimInfo = mkOption {
+    type = (types.nullOr types.attrs) // {
+      # This will create an attrset of the form:
+      #
+      # { path.to.plugin.name = <info>; }
+      #
+      #
+      # Where <info> is an attrset of the form:
+      # {
+      #   file = "path";
+      #   description = null or "<DESCRIPTION>";
+      #   url = null or "<URL>";
+      # }
+      merge =
+        _: defs:
+        lib.foldl'
+          (
+            acc: def:
+            lib.recursiveUpdate acc (
+              setAttrByPath def.value.path {
+                inherit (def) file;
+                url = def.value.url or null;
+                description = def.value.description or null;
+              }
+            )
+          )
+          {
+            plugins = { };
+            colorschemes = { };
+          }
+          defs;
+    };
+    internal = true;
+    default = null;
+    description = ''
+      Nixvim related information on the module
+    '';
+  };
+}