Cannot call service: "'MyPlugin' not found" error
I found the issue. The author of the tutorial I copied my first code from made the unfortunate choice to not use a proper plugin class:

class Plugin extends \craft\base\Plugin { ... }


To fix the problem, I replaced this line with

use Craft;
use craft\base\Plugin;
class Yutulic extends Plugin { ... }


and had to add a reference to the class to my controller and service modules:

use ns\yutulic\Yutulic;