using rel="alternate" rather than multiple sitemaps by language context (Drupal 8)

Such feature is present in the simple_sitemap module. I found it after spent several hours trying to make xmlsitemap run on a multilingual site with no success.

I think I can make this work in a backwards compatible way. I can rely implementing hook_xmlsitemap_element_alter() on behalf of content_moderation to inject the alternate links.

We will likely want to get rid of the language context eventually, likely in 2.0, since removing it would mean we need to inspect everyone's sitemap entities again, remove the context and then delete any duplicate sitemap entities.

I would *LOVE* some help testing this out.

I also wanted this to work automatically if there is only one sitemap created before the language module was enabled, and so the sitemap entity does not yet have a language context. This should help solve #3006297: When language module is installed, update sitemaps with language context.

Fixing error in the latest patch.

Better version that merges in the default contexts when loading the sitemap entities, instead of using language_xmlsitemap_context_alter(). That will make it compatible with all possible contexts. Still need to figure out what's going on with the test coverage.

Last patch did not have the cached changes only.

OMG it appears to be working all correctly!

<?xml version="1.0" encoding="UTF-8"?>
<?xml-stylesheet type="text/xsl" href="/sitemap.xsl"?>
<urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9">
<url><loc>https://local.drupal.com/</loc><changefreq>daily</changefreq><priority>1.0</priority></url>
<url><loc>https://local.drupal.com/node/2</loc><lastmod>2019-12-03T23:39Z</lastmod><changefreq>daily</changefreq>
<xhtml:link rel="alternate" hreflang="es" href="https://local.drupal.com/es/node/2" />
<xhtml:link rel="alternate" hreflang="fr" href="https://local.drupal.com/fr/node/2" />
</url>
<url><loc>https://local.drupal.com/user/3</loc><lastmod>2019-11-04T15:14Z</lastmod><changefreq>monthly</changefreq></url>
</urlset>

We were missing the xmlns:xhtml="http://www.w3.org/1999/xhtml attribute on the root XML, so let's just go ahead and add it globally.

Fixing test that no longer assumes we can set a sitemap to language undefined, that seems a bit silly.

I added support for ensuring that we also remove any invalid contexts in \Drupal\xmlsitemap\XmlSitemapStorage::doLoadMultiple(), fixed the empty context message on the sitemap edit form when there are contexts available, and also added alternate links for the homepage link.

Grr, new last one was just the same.

Reduced repeated code by adding an internal function for adding alternate links.

Okay I feel really good about this. I'm going to go ahead and commit this to 8.x-1.x and I would *really* like some testing on the 8.x-1.x-dev branch from anyone who can!

I have found that if a translated node, is also linked in a menu_link_content item that is not translated, this means that likely the menu_link_content item is read first, and none of the alternate links are added in this case. I'll file a follow-up issue for this.

This should help resolve it if we encounter a menu_link_content item for a node vs the node first in the xmlsitemap table in generation.

Fixing coding standards.

Tested and committed #23 to 8.x-1.x.

This has now been included in the 8.x-1.0-rc1 release, feedback and testing results would be welcome.