It seems like many plugins (GitHub Multibranch and Bitbucket Multibranch at the very minimum) are using `onCheckout` event to send build notifications.

Sometimes it's just doesn't work due to unrelated bugs related to all the complexity with multi-scm pipelines, for instance - https://issues.jenkins.io/browse/JENKINS-66040.

Sometimes when it does - it might be too late. Imagine your groovy lib or custom project factory does a whole bunch of stuff to generate an actual pipeline - by the time it does a first real `scm checkout` it might be 20-30 or more seconds after the build have started.

I am not sure if calling `onCheckout` on all SCM access would be a good idea, but maybe a new event to the SCMListener would do. Consumer plugins could then use this event to send out notifications earlier in the game.