[macro] disallow redefining modules

[kLabz]

Also note that this was allowed in Haxe 4 too.

• Implement haxe.macro.CompilationServer.invalidateModule() to give a way to actually replace a module (that is not loaded yet)
• Fix the stuff that makes some tests break.. 😅
• (?) Move the hook to somewhere it belongs

[Simn]

This whole hook approach doesn't make sense to me. There should be exactly two cases when dealing with modules via define_module and define_type:

1. The module already exists in the current compilation context (com.module_lut). In this case we give a redefinition error.
2. The module doesn't exist in the current compilation context. In this case we add it and dirty any cached modules matching it. This will cause any of its dependents to pick up the new module.

What exactly isn't working with this general approach?