plugins/completion: move to by-name

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

tests/test-sources/plugins/cmp/cmp-async-path.nix

@@ -0,0 +1,16 @@
+{
+  example = {
+    plugins.cmp = {
+      enable = true;
+      settings.sources = [
+        {
+          name = "async_path";
+          option = {
+            trailing_slash = false;
+            label_trailing_slash = true;
+          };
+        }
+      ];
+    };
+  };
+}