Annotations without class import?
Drupal's annotation system is based on the Doctrine PHP library, and not on Symfony like the rest of Drupal. 

Doctrine uses a global annotation registry to map classes to files (rather than an autoloader). This page contains the list of classes in that registry.

This issue from 2013 shows the Drupal maintainers' decision to remove the need to import annotation classes.

You wouldn't know this by looking at the official Drupal documentation -- they say the annotation classes are loaded "magically".