Fixed package XML deployment for root directory targets

Trying to deploy `Mage_All_Latest` with the composer installer results in a number of broken symlinks because the installer does not appear to handle files destined for the root directory correctly. Instead of deploying e.g. `./index.php` to `./index.php`, it is deployed to `././index.php`.

This PR fixes this behavior — however I am not sure if there is a better/more compatible way to do this. Feedback very welcome!