Convert StomachBehavior to a component/system + rejig body namespaces (#5249)

* Convert StomachBehavior to a component/system + rejig body namespaces

* test

* slightly more namespace changes

* remove

* Hello?????

* fuck you github test runner

* reviews

* oobsy!

Content.IntegrationTests/Tests/Body/MechanismBehaviorEventsTest.cs

@@ -3,7 +3,6 @@ using System.Linq;
 using System.Threading.Tasks;
 using Content.Server.Body.Behavior;
 using Content.Shared.Body.Components;
-using Content.Shared.Body.Mechanism;
 using Content.Shared.Body.Part;
 using NUnit.Framework;
 using Robust.Shared.GameObjects;