Move commands module into bevy::ecs::world

[JoJoJet]

What problem does this solve or what need does it fill?

Currently, commands live in bevy::ecs::system, as they are referred to a "system commands". It would make more sense to put this module in world, as commands are an abstraction for efficiently applying mutations to a world. Systems are only a consumer of this API.

What solution would you like?

Command and CommandQueue should go into bevy::ecs::world. Commands and ParallelCommands should stay in bevy::ecs::system.

What alternative(s) have you considered?

None

Additional context

None

[targrub]

I'll work on this.

[JoJoJet]

On second thought it would be good to hold off until after #10756

[targrub]

Now that #10756 is merged, shall I proceed with this proposal?

[JoJoJet]

Please!

[targrub]

The need to have a migration for this change will annoy people, I suspect.

From:

use bevy_ecs {
    system::{Command, CommandQueue, Commands},
    world::World,
}

To:

use bevy_ecs {
    system::Commands,
    world::{Command, CommandQueue, World},
}