Built-in classes should not be ignored when specifically requested in rules

[carlosas]

When the option ignore_built_in_classes is enabled, and there is a rule like:

return PHPat::rule()
  ->classes(Selector::namespace('App\Whatever'))
  ->shouldImplement()
  ->classes(Selector::classname(\JsonSerializable::class));

PHPat reports:

App\Whatever\MyClass should implement JsonSerializable

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Built-in classes should be removed from the violations, instead of the nodes to be checked.