code: public function classBySource(ClassName $className, $source)
      {
          $this->typeCheck->classBySource(func_get_args());
  
          foreach ($this->classesBySource($source) as $classDefinition) {
              if ($classDefinition->className()->string() === $className->toAbsolute()->string()) {
                  return $classDefinition;
              }
          }
  
          throw new Exception\UndefinedClassException($className);
      }
docs: @param ClassName $className
  @param string    $source
  
  @return ClassDefinition
  @throws Exception\UndefinedClassException