EZP-21408: Add composer autoloads in ezpgenerateautoloads.php in pure legacy context

This patch adds the ability to use the Zeta Components installed through composer in a pure eZ Publish legacy install without the need to download the eZ Components into `lib/ezc` or to install them through PEAR.

Without this patch, the call of `php bin/php/ezpgenerateautoloads.php -e` would stop with the following error:

```bash
Warning: require(Base/src/base.php): failed to open stream: No such file or directory in .../ezpublish-legacy/bin/php/ezpgenerateautoloads.php on line 45
```