Avoid "No PHP exe defined" when PHP exe is available via ext.p.

If the user hasn't selected a PHP executable in the Composer's
preference page, then the Symfony project wizard shows "No PHP
executable defined" validation error and new project cannot be created.
However, there might be a PHP executable configured by another plugin
using the org.eclipse.php.debug.core.phpExe extension point. In this
case such PHP executable can do the job and this error should not be
displayed.

The wizard validator should try to get any PHP executable if one is not
available by just reading the preferences.