Make the `BuildSystem` classes use the machinery from the `variables`.

[jjotero]

The BuildSystem classes have the same structure as the RegressionTest classes used to before the variables were introduced. We could spin out a lighter version of the RegressionTestMeta and make use of the variables also in these BuildSystem classes. That would standardise the use of the required keyword throughout ReFrame, instead of each module defining their own _UndefinedType.

#2027 shows why this would be beneficial

[jjotero]

From a preliminary look into this, this would require to move the the parameter and variable injection from the __new__ method in the RegressionTest class to the __call__ method of the RegressionTestMeta. Otherwise, each class using this metaclass would be forced to explicitly call the injection every time, which doesn't make much sense. For the same effort, we can also move the hook decoration into RegressionTestMeta's __call__ method, which would look as follows:

class Meta(type):
    ...
    def __call__(cls, *args, **kwargs):
        obj = cls.__new__(cls, *args, **kwargs)
        
        # Inject the parameters and variables, and also decorate the hooks
        cls._rfm_var_space.inject(obj, cls)                                                                                
        cls._rfm_param_space.inject(obj, cls, _rfm_use_params)
        for stage in _PIPELINE_STAGES:
            cls._add_hooks(stage)    

        obj.__init__(*args, **kwargs)
        return obj

The __rfm_init__ method from RegressionTest could be then attached as a "pre-init" hook. The prefix and name logic in __new__ could go also into __rfm_init__, except for the old naming of the @parameterized_test decorator. In short, this should be done after the @parameterized_test deco is out.

Another bonus of moving all this stuff in here is that __call__ does not get "called" during a deep-copy, but __new__ does.