sfence

PR prepared for review. Added `core.objects_by_guid` table to Lua API. The command `/unittests` should be called two times to verify if `core.objects_by_guid` is set and cleared.

Rebased and merged to one commit for easier rebasing in the future.

> Alternatively we can add some hooks `before_mod_load`, `after_mod_load` (or something, not good at naming) that are executed at those stages. Games can then implement these hooks in one or...

@appgurueu Thanks for the feedback. Should be testable in C++ unittests. Lua solutions take less effort to create. But C++ unittest will be probably better for it. I will look...

> just some a thought, not 100% > > relating to the keys in game.conf, would it make sense to have them be something like `load_first_mod = modname` and `load_last_mod...

> Fair enough. I thoughts since games control their mods, they can control the sets of mods that do that. > If this is a better approach let's do this...

> Each world has a list of enabled mods. Why not set the order with that list with enabled mods at the top of the list with the user able...

I have compiled Minetest on my MacBook M2 and run `./bin/minetest --run-unittests` command and I do not see the problem from the GitHub Macos test. So hopefully, it is not...

> Wasn't there some discussion to move the entire logic to Lua? Yes, in #14125. But this solution should target to allow full backward compatibility and moving logic to Lua...

> Because, contrary to HPs, it's not something the majority of games need. It should be eradicated from the engine imho, it's only a burden for core devs I understand....