Introduce base-class based sub-class discovery [5.3.0-B1]

I've made quite a few improvements to the Phabricator tool recently and it uses quite different approach to autoloading:

you never register a class specifically or place it under namespace
the tokenizer discovers all PHP files in project and records this:
- what classes/interfaces/traits are created in each file
- what class/interface/trait extends other class/interface/trait
extensively using sub-classing
has method to return all class instances that sub-class given class (directly or indirectly)

I believe latter can be a huge win for In-Portal before:

you only register class once in unit config and class itself then knows how to integrate with system
no need to create several configuration options for thing, e.g. (all things below aren't yet committed):
- we register router class (with on configuration option) and specify it's priority (with another configuration option) - Routing Ideas
- we register CLI command class (with on configuration option) and specify that it should be used (with another configuration option) - [console] Use Symfony Console Component for Automation [5.3.0-B1]
- we register scheduled task class (with on configuration option) and specify it's run schedule (with another configuration option) - Alexander Obuhovich's Home

To make this happen we need to, at class registration time (when unit config cache is rebuilt), also create records that will:

INP-1429 - Getting issue details... STATUS