Gem::Commands::SetupCommand#install_lib

# File tmp/rubies/ruby-4.1.0-preview1/lib/rubygems/commands/setup_command.rb, line 284
def install_lib(lib_dir)
  libs = { "RubyGems" => "lib" }
  libs.each do |tool, path|
    say "Installing #{tool}" if @verbose

    lib_files = files_in path

    Dir.chdir path do
      install_file_list(lib_files, lib_dir)
    end
  end
end