Use ::class instead of string literals in plugin managers

Problem/Motivation

I found plenty of plugin managers which use string literals for class names.
These should use ::class instead.

Steps to reproduce

Proposed resolution

Use ::class to refer to classes and interfaces.
For now the scope is plugin managers.

In case of name clash of the class alias (e.g. attribute vs annotation) we can use other shortcuts like `Annotation\Something`, where we might import the respective Annotation namespace.

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet