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/CharacterAppearance/HumanoidVisualLayersExtension.cs

@@ -1,5 +1,6 @@
 using System;
 using System.Collections.Generic;
+using Content.Shared.Body.Components;
 using Content.Shared.Body.Part;
 
 namespace Content.Shared.CharacterAppearance