Epic #47018: Implement Composer support and clean package manager API
Allow the activation of packages during runtime
Due to the nature of the Flow Package Manager, packages cannot be activated and directly used during runtime. Before the Package Manager it was possible to activate/deactivate extensions in AdditionalConfiguration.php under certain custom conditions.
This should be made possible again, but with the limitations that packages can just be activated for runtime usage, not deactivated and cannot introduce own class alias mappings.
The ExtensionManager can use this too to load a package after activation until the end of the request and perform further updates of the caching framework and category api.