Gem::Installer#load_plugin

# File tmp/rubies/ruby-4.1.0-preview1/lib/rubygems/installer.rb, line 1059
def load_plugin
  specs = Gem::Specification.find_all_by_name(spec.name)
  # If old version already exists, this plugin isn't loaded
  # immediately. It's for avoiding a case that multiple versions
  # are loaded at the same time.
  return unless specs.size == 1

  plugin_files = spec.plugins.filter_map do |plugin|
    path = File.join(@plugins_dir, "#{spec.name}_plugin#{File.extname(plugin)}")
    path if File.exist?(path)
  end
  Gem.load_plugin_files(plugin_files) unless plugin_files.empty?
end