Issue #3045063: Change redirect hash to match the Redirect module

Builds the duplicate pre-check hash the way Redirect::preSave() does: from the source path with the leading slash removed, and the language set on the redirect entity. The check used to hash the destination path, so it never found the existing redirect and the second save hit the unique hash index. w.drupal's re-roll from #7 supplies the hashing; the second commit drops the setLanguage() call queried in #12, reads the language off the entity instead, and adds two kernel tests that save the same redirect twice, one of them with a non-default language.