Add leading slash to js-modules within cordova_plugins.json

This fixes loading of modules from HTML files not in project root.

src/prepare.js

@@ -120,7 +120,7 @@ module.exports = function handlePrepare(project_dir, platform, plugins_dir) {
 
                 // Prepare the object for cordova_plugins.json.
                 var obj = {
-                    file: path.join('plugins', plugin_id, module.attrib.src),
+                    file: path.join('/plugins', plugin_id, module.attrib.src),
                     id: moduleName
                 };