Fix the legacy registered hooks being overwritten by the annotation ones

[qzminski]

If you have any hooks that are registered in the legacy way:

// config/config.php
$GLOBALS['ISO_HOOKS']['generateProduct'][] = […];

They will get overwritten if there is at least one hook registered with annotations:

use Isotope\ServiceAnnotation\IsotopeHook;

/**
 * @IsotopeHook("generateProduct")
 */
class MyListener {
    // …
}

This pull request aims to fix that.

[aschempp]

@qzminski can you please check if everything still works as expected with my changes?

[qzminski]

I can confirm it works as expected.