Fix plugin modules not loading

Fix the path to plugin modules using the old plugins dir instead of
resolving to the actual path of the plugin, which in turn caused no
plugin modules to be loaded.

packages/slave/slave.js

@@ -384,7 +384,8 @@ class Instance extends link.Link{
 		// Find plugin modules to patch in
 		let modules = new Map();
 		for (let [pluginName, plugin] of this.plugins) {
-			let modulePath = path.join("plugins", pluginName, "module");
+			let pluginPackagePath = require.resolve(path.posix.join(plugin.info.requirePath, "package.json"));
+			let modulePath = path.join(path.dirname(pluginPackagePath), "module");
 			if (!await fs.pathExists(modulePath)) {
 				continue;
 			}