Refactor autoload.php with a constant conditional

[siwinski]

This is up for debate.

Something didn't feel right about the following to me:

if (!class_exists('Fedora\\Autoloader\\Autoload', false)) {
    require_once '/usr/share/php/Fedora/Autoloader/autoload.php';
}

This change allows autoload.php to be fully loaded (even if class Fedora\\Autoloader\\Autoload happened to be loaded already) successfully multiple times without any PHP errors. The constant is just for conditionals and to prevent multiple addPsr4() calls. If we wanted to put the loading conditional in other files, we could use:

if (!defined('FEDORA_AUTOLOADER')) {
    require_once '/usr/share/php/Fedora/Autoloader/autoload.php';
}

[remicollet]

I'm fine with this, cleaner.

[remicollet]

Just for memory, the only known case where double include of this file happens: during the php-fedora-autoloder package build (included from phpunit main autoloader + by tests bootstrap).