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.

wrappers/_shared.nix

@@ -1,23 +1,37 @@
 helpers:
 { lib, config, ... }:
 let
-  inherit (lib) mkOption mkOptionType;
+  inherit (lib)
+    isAttrs
+    listToAttrs
+    map
+    mkOption
+    mkOptionType
+    ;
   cfg = config.programs.nixvim;
+  extraFiles = lib.filter (file: file.enable) (lib.attrValues cfg.extraFiles);
 in
 {
   helpers = mkOption {
     type = mkOptionType {
       name = "helpers";
       description = "Helpers that can be used when writing nixvim configs";
-      check = builtins.isAttrs;
+      check = isAttrs;
     };
     description = "Use this option to access the helpers";
     default = helpers;
   };
 
-  configFiles =
-    (lib.mapAttrs' (_: file: lib.nameValuePair "nvim/${file.path}" { text = file.content; }) cfg.files)
-    // (lib.mapAttrs' (
-      path: content: lib.nameValuePair "nvim/${path}" { text = content; }
-    ) cfg.extraFiles);
+  # extraFiles, but nested under "nvim/" for use in etc/xdg config
+  configFiles = listToAttrs (
+    map (
+      { target, source, ... }:
+      {
+        name = "nvim/" + target;
+        value = {
+          inherit source;
+        };
+      }
+    ) extraFiles
+  );
 }