plugins/completion: move to by-name

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

tests/test-sources/plugins/by-name/coq-thirdparty/default.nix

@@ -0,0 +1,29 @@
+{
+  # Empty configuration
+  empty = {
+    plugins.coq-thirdparty.enable = true;
+  };
+
+  example = {
+    plugins.coq-thirdparty = {
+      enable = true;
+
+      sources = [
+        {
+          src = "nvimlua";
+          short_name = "nLUA";
+        }
+        {
+          src = "vimtex";
+          short_name = "vTEX";
+        }
+        {
+          src = "copilot";
+          short_name = "COP";
+          accept_key = "<c-f>";
+        }
+        { src = "demo"; }
+      ];
+    };
+  };
+}