Add original yaml_backend for Hiera 1.x.
user
```ruby
class Hiera
  module Backend
    class Yaml_backend
      def initialize(cache=nil)
        require 'yaml'
        Hiera.debug("Hiera YAML backend starting")

        @cache = cache || Filecache.new
      end

      def lookup(key, scope, order_override, resolution_type)
        answer = nil

        Hiera.debug("Looking up #{key} in YAML backend")

        Backend.datasourcefiles(:yaml, scope, "yaml", order_override) do |source, yamlfile|
          data = @cache.read_file(yamlfile, Hash) do |data|
            YAML.load(data) || {}
          end

          next if data.empty?
          next unless data.include?(key)

          # Extra logging that we found the key. This can be outputted
          # multiple times if the resolution type is array or hash but that
          # should be expected as the logging will then tell the user ALL the
          # places where the key is found.
          Hiera.debug("Found #{key} in #{source}")

          # for array resolution we just append to the array whatever
          # we find, we then goes onto the next file and keep adding to
          # the array
          #
          # for priority searches we break after the first found data item
          new_answer = Backend.parse_answer(data[key], scope)
          case resolution_type
          when :array
            raise Exception, "Hiera type mismatch: expected Array and got #{new_answer.class}" unless new_answer.kind_of? Array or new_answer.kind_of? String
            answer ||= []
            answer << new_answer
          when :hash
            raise Exception, "Hiera type mismatch: expected Hash and got #{new_answer.class}" unless new_answer.kind_of? Hash
            answer ||= {}
            answer = Backend.merge_answer(new_answer,answer)
          else
            answer = new_answer
            break
          end
        end

        return answer
      end

      private

      def file_exists?(path)
        File.exist? path
      end
    end
  end
end
```
assistant