module Outer
  class Inner; end
  class AnotherInner; end
end
class C
  import Outer: [:Inner, :AnotherInner]
  def f
    Inner
  end
end

class Class
  def import(constants)
    @imported_constants =
      (@imported_constants || {}).merge Hash[
        constants.flat_map { |namespace, names|
          [*names].map { |name| [name.to_sym, "#{namespace}::#{name}"] }
        }]
  end

  def const_missing(name)
    const_set name, eval(@imported_constants[name] || raise)
  end
end

module Outer
  class Inner; end
  class AnotherInner; end
end

class C
  include Outer

  def f
    Inner
  end
end

C.new.f # => Outer::Inner