Gem::Installer#load_plugin

# File tmp/rubies/ruby-3.4.1/lib/rubygems/installer.rb, line 1019
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.map do |plugin|
    File.join(@plugins_dir, "#{spec.name}_plugin#{File.extname(plugin)}")
  end
  Gem.load_plugin_files(plugin_files)
end