Fail to restore plugins with relative path

1. Create a cordova project
2. Add android platform
3. Add a local plugin using a relative path: cordova plugin add myplugins/testplugin
4. Delete directories platforms/ and plugins/
5. Prepare android: cordova prepare android

Cordova will fail to restore the local plugin. The plugin has been added to package.json using the spec file:myplugins/testplugin and the restoration utility is unable to handle such URL. Note that config.xml contains the spec myplugins/testplugin (that is, without the file: scheme), but the one in package.json takes precedence.