plugins/completion: move to by-name

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

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

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