Fixed the extraction of the zip archive

The extracted Symfony folder is now longer kept inside the project alongside a mirror of its content.
Closes #9

This also helps improving performance for #8. Renaming a folder is much faster than mirroring it (which requires copying all its content recursively). Being able to rename the ``Symfony`` folder to ``$projectDir`` directly would likely be even faster, but it would involve extracting the zip archive to a separate location (you cannot rename a folder to the name of its parent) which is why it is not done here