plugins/completion: move to by-name

- Move nvim-cmp to plugins/cmp
- Move other completion plugins to plugins/by-name

plugins/cmp/sources/cmp-fish.nix

@@ -0,0 +1,22 @@
+{
+  lib,
+  config,
+  pkgs,
+  helpers,
+  ...
+}:
+with lib;
+let
+  cfg = config.plugins.cmp-fish;
+in
+{
+  meta.maintainers = [ maintainers.GaetanLepage ];
+
+  options.plugins.cmp-fish = {
+    fishPackage = lib.mkPackageOption pkgs "fish" {
+      nullable = true;
+    };
+  };
+
+  config = mkIf cfg.enable { extraPackages = [ cfg.fishPackage ]; };
+}