Fixing aliasing namespaced classes

This fixes #5179. We currently have 2 places where class aliases are registered, once in /libraries/classmap.php and once in /libraries/cms.php. I decided to only call class_alias() directly for JAdministrator, JSite and JRegistry, since those 3 classes are classes that are passed between extensions and objects. This always loads those 3 classes on every pageload, but I hope that downside is negligible. All other classes are extremely likely to be instantiated first and then checked against, which is why I doubt that we need to care for those separately. Basically, all JRegistry* classes are only used internally of JRegistry and JStringInflector seems to be a class that most likely is not created by the CMS and then used by an extension. JStringNormalise is abstract anyway, so this should not be a problem either.