When upgrading to eZ Publish 5.4.5 using composer update, the following exception is thrown (on cache clear):

  [eZ\Bundle\EzPublishCoreBundle\ApiLoader\Exception\InvalidSearchEngine]
  Invalid search engine 'legacy'. Could not find a service tagged as 'ezpublish.searchEngine' with alias 'legacy'.

The solution appears to be adding EzPublishLegacySearchEngineBundle() to the kernel's registerBundles() method, however this is not mentioned in https://doc.ez.no/display/EZP/5.4.x+Update+Instructions