feat(17-03): full sitemap generation, settings, hooks and import commands

- generator in Definition::addItemToSet order: 50000 URL cap, lastmod from
  the record or the updates manifest (regular file, 5 MiB cap, fail safe),
  BeforeAddItem (skip) and AddItem (extra XML) festival events, tier or item
  priority, per-alternate expansion of list type children
- priority tiers from golem15.sitemap.priority_tiers, compiled at boot,
  defaults equal to the PHP getPriorityForUrl rules
- frontend_url rewrite of loc, xhtml:link href and the stylesheet href;
  GET /sitemap.xsl serves the PHP stylesheet byte for byte
- golem15_sitemap_settings singleton and Settings model
- sitemap:import (PHP ImportCommand) and sitemap:import-winter with the
  read-only Winter export SQL
- urlto and full goldens generated from PHP; TestSitemapImportSmoke

tests/php/gen-goldens.php

@@ -33,8 +33,8 @@
  * A registry type's resolved item is given in the Go shape (one item per
  * record with ordered alternates); this script turns it into what the PHP
  * headless resolvers returned: a list type (no url) yields one entry per
- * alternate for each child, any other type its children with their default
- * URL.
+ * alternate for each child (a child without alternates once), any other type
+ * its children with their default URL.
  */
 
 $vendor = getenv('WINTER_VENDOR');
@@ -194,7 +194,7 @@ class GoldenSitemap
             $info['items'] = [];
             foreach ($r['items'] as $child) {
                 $childInfo = $this->phpResolved($child, false);
-                if ($listChildren) {
+                if ($listChildren && $childInfo['alternateLinks']) {
                     foreach ($childInfo['alternateLinks'] as $locale => $href) {
                         $perLocale = $childInfo;
                         $perLocale['url'] = $href;