plugins/completion: move to by-name

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

plugins/by-name/coq-thirdparty/default.nix

@@ -0,0 +1,77 @@
+{
+  lib,
+  helpers,
+  config,
+  pkgs,
+  ...
+}:
+with lib;
+let
+  cfg = config.plugins.coq-thirdparty;
+in
+{
+  options.plugins.coq-thirdparty = {
+    enable = mkEnableOption "coq-thirdparty";
+
+    package = lib.mkPackageOption pkgs "coq-thirdparty" {
+      default = [
+        "vimPlugins"
+        "coq-thirdparty"
+      ];
+    };
+
+    sources = mkOption {
+      type = types.listOf (
+        types.submodule {
+          freeformType = types.attrs;
+
+          options = {
+            src = mkOption {
+              type = types.str;
+              description = "The name of the source";
+            };
+
+            short_name = mkOption {
+              type = types.nullOr types.str;
+              description = ''
+                A short name for the source.
+                If not specified, it is uppercase `src`.
+              '';
+              example = "nLUA";
+              default = null;
+            };
+          };
+        }
+      );
+      description = ''
+        List of sources.
+        Each source is a free-form type, so additional settings like `accept_key` may be specified even if they are not declared by nixvim.
+      '';
+      default = [ ];
+      example = [
+        {
+          src = "nvimlua";
+          short_name = "nLUA";
+        }
+        {
+          src = "vimtex";
+          short_name = "vTEX";
+        }
+        {
+          src = "copilot";
+          short_name = "COP";
+          accept_key = "<c-f>";
+        }
+        { src = "demo"; }
+      ];
+    };
+  };
+
+  config = mkIf cfg.enable {
+    extraPlugins = [ cfg.package ];
+
+    extraConfigLua = ''
+      require('coq_3p')(${helpers.toLuaObject cfg.sources})
+    '';
+  };
+}