Improve EZP-23351: Honor relative flag on legacy:assets_install for legacy wrappers

Absolute path in these scripts means they are also broken in distributions.
As we use relative by default, honoring this flag fixes this.
Improve EZP-23489: Make assets install scripts smarter with symlinks

The corresponding 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)
Merge pull request #1029 from ezsystems/dx_symlinks

[DX] Make legacy asset commands smarter with symlinks & wrapper honor relative