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.Shared/Body/Systems/MechanismSystem.cs

@@ -0,0 +1,20 @@
+using Content.Shared.Body.Components;
+using JetBrains.Annotations;
+using Robust.Shared.GameObjects;
+
+namespace Content.Shared.Body.Systems
+{
+    [UsedImplicitly]
+    public class MechanismSystem : EntitySystem
+    {
+        public override void Update(float frameTime)
+        {
+            base.Update(frameTime);
+
+            foreach (var mechanism in EntityManager.EntityQuery<SharedMechanismComponent>(true))
+            {
+                mechanism.Update(frameTime);
+            }
+        }
+    }
+}