Make ezpublish:legacy:assets_install & ezpublish:legacybundles:install_extensions smarter with symlinks

Mostly same change as in:
https://github.com/symfony/symfony/pull/11312

The Symfony change was not backported, so this works best in combination with Symfony 2.6,
but will also slightly benefit users on Symfony 2.3 (at least for these commands)