modules: refactor extraFiles

Moved `extraFiles` from `modules/output.nix` into its own file `modules/files.nix`.

Users should now assign text to a `text` attribute, however they could
also assign a file path to a `source` attribute instead.

The old method of directly assigning a string still works, and is
coerced to the new type along with a deprecation warning.

modules/top-level/files/default.nix

@@ -1,6 +1,7 @@
 {
   pkgs,
   config,
+  options,
   lib,
   helpers,
   ...
@@ -49,8 +50,8 @@ in
 
   config =
     let
-      inherit (config) files;
-      concatFilesOption = attr: lib.flatten (lib.mapAttrsToList (_: builtins.getAttr attr) files);
+      extraFiles = lib.filter (file: file.enable) (lib.attrValues config.extraFiles);
+      concatFilesOption = attr: lib.flatten (lib.mapAttrsToList (_: builtins.getAttr attr) config.files);
     in
     {
       # Each file can declare plugins/packages/warnings/assertions
@@ -59,12 +60,38 @@ in
       warnings = concatFilesOption "warnings";
       assertions = concatFilesOption "assertions";
 
+      # Add files to extraFiles
+      extraFiles = lib.mkDerivedConfig options.files (
+        lib.mapAttrs' (
+          _: file: {
+            name = file.path;
+            value.source = file.plugin;
+          }
+        )
+      );
+
       # A directory with all the files in it
-      filesPlugin = pkgs.buildEnv {
-        name = "nixvim-config";
-        paths =
-          (lib.mapAttrsToList (_: file: file.plugin) files)
-          ++ (lib.mapAttrsToList pkgs.writeTextDir config.extraFiles);
-      };
+      # Implementation based on NixOS's /etc module
+      filesPlugin = pkgs.runCommandLocal "nvim-config" { } ''
+        set -euo pipefail
+
+        makeEntry() {
+          src="$1"
+          target="$2"
+          mkdir -p "$out/$(dirname "$target")"
+          cp "$src" "$out/$target"
+        }
+
+        mkdir -p "$out"
+        ${lib.concatMapStringsSep "\n" (
+          { target, source, ... }:
+          lib.escapeShellArgs [
+            "makeEntry"
+            # Force local source paths to be added to the store
+            "${source}"
+            target
+          ]
+        ) extraFiles}
+      '';
     };
 }

modules/top-level/files/submodule.nix

@@ -14,9 +14,14 @@
       internal = true;
     };
   };
-  config = {
-    path = name;
-    type = lib.mkDefault (if lib.hasSuffix ".vim" name then "vim" else "lua");
-    plugin = pkgs.writeTextDir config.path config.content;
-  };
+  config =
+    let
+      derivationName = "nvim-" + lib.replaceStrings [ "/" ] [ "-" ] name;
+    in
+    {
+      path = lib.mkDefault name;
+      type = lib.mkDefault (if lib.hasSuffix ".vim" name then "vim" else "lua");
+      # No need to use mkDerivedConfig; this option is readOnly.
+      plugin = pkgs.writeText derivationName config.content;
+    };
 }