You can load your plugin dependencies from within the plugin that needs it:
$returnValue = kirby()->plugin('test-plugin');
The test-plugin plugin won’t be loaded again later, so using that function as often as you need doesn’t cause any problems. The return value is stored for later use.
Thanks a lot!
Since I’m regularly helping developing Kirby in my spare time for I guess three years now, I have been digging through the Kirby code a lot.