[bugfix] Fix the call order on the __set_name__ hook for built-in variables

[jjotero]

When declaring a variable with the variable built-in, the corresponding __set_name__ hook was only called if needed by the __getitem__ method of the metaclass. This is because the variables are temporarily hidden away in their own namespace, and the __set_name__ hooks from these variables don't get called by the base metaclass type.

Hence, it is a bit counterintuitive (if not just plainly wrong) that things get set when calling the __getitem__ method; so now the __set_name__ hook is called unconditionally for all variables in the __setitem__ method of the metaclass.