Follow-up to #1937600: Determine what services to register in the new Drupal class
We need to decide what we want to do with plugin managers, some of them are quite frequently accessed.
My suggestion:
1. Enforce the naming pattern that afaik all or most are already using, which is plugin.manager.$something
2. Add a Drupal::pluginManager($something)
3. For those that are accessed very frequently and provide custom methods, e.g. entityManager(), add a specific method so that we have type hints for the additional methods.