FEATURE: Allow to preview non-site packages

[kdambekalns]

I am building a component library in it's own package. That package is not a neos-site, but a simple neos-package. As such it does not appear in the package selector in Monocle.

Since offering all packages is useless and checking all packages for potentially providing components, I'd suggest a way to register packages for inclusion through Monocle's settings.

[bwaidelich]

FYI: This one is related to #109 and #110 slightly

[bwaidelich]

I had a quick look, but this would probably require a bigger change since the PackageKeyTrait is used in quite a few places.
I got it to work for our case with a little AOP aspect (in case someone else needs this):

<?php
declare(strict_types=1);
namespace Some\Package\Aspects;

use Neos\Flow\Annotations as Flow;
use Neos\Flow\Aop\JoinPointInterface;

/**
 * @Flow\Aspect
 */
final class MonoclePackageKeyTraitAspect
{

    /**
     * Aspect that prepends the active site package list with "Some.Package" so that this package appears in the
     * site selector of the Monocle Preview
     *
     *
     * @Flow\Around("method(Sitegeist\Monocle\Controller\ApiController->getActiveSitePackageKeys())")
     * @param JoinPointInterface $joinPoint
     * @return array
     */
    public function addPackageToActiveSitePackageKeys(JoinPointInterface $joinPoint): array
    {
        $activeSitePackages = $joinPoint->getAdviceChain()->proceed($joinPoint);
        array_unshift($activeSitePackages, 'Some.Package');
        return $activeSitePackages;
    }

}