`wp plugin` fails when plugin bundles another plugin

Create a plugin inside another plugin:

```
$ wp plugin scaffold --skip-tests plugin1
$ wp plugin status plugin1
$ wp plugin scaffold --skip-tests plugin1/demo
$ wp plugin scaffold --skip-tests demo
$ cd $(wp plugin path)
$ mv demo/ plugin1/
```

Now try to activate the parent:

```
$ wp plugin activate plugin1
Warning: Could not activate the 'plugin1' plugin.
```

The root cause is that WP-CLI picks up `plugin1/demo/demo.php`, whereas Core doesn't check a plugin's subdirectories.