use `require_relative` to load Jekyll classes (#6609)

Merge pull request 6609

lib/jekyll/liquid_renderer.rb

 # frozen_string_literal: true
-require "jekyll/liquid_renderer/file"
-require "jekyll/liquid_renderer/table"
+require_relative "liquid_renderer/file"
+require_relative "liquid_renderer/table"
 module Jekyll
   class LiquidRenderer