Forbid top level route sitemap.xml

According to the official documentation in https://www.sitemaps.org/protocol.html#location, this is what it says for the location of the sitemap:

The location of a Sitemap file determines the set of URLs that can be included in that Sitemap. A Sitemap file located at http://example.com/catalog/sitemap.xml can include any URLs starting with http://example.com/catalog/ but can not include URLs starting with http://example.com/images/.

Also, in Google's General sitemap guidelines:

A sitemap can be posted anywhere on your site, but a sitemap affects only descendants of the parent directory. Therefore, a sitemap posted at the site root can affect all files on the site, which is where we recommend posting your sitemaps.

Therefore, we need to make available the top-level route /sitemap.xml and /sitemap.xml.gz (in case we want to serve it compressed).

This might affect namespaces with the path sitemap.xml but we need to make it available. We can make up a custom sitemap name almost impossible to collide with existing namespaces but it's better to use the default name because most of the time that default route is used to discover it and we use a made-up one it won't be possible.

The general idea is to use the sitemap.xml file as an index and the place any other necessary sitemaps in /public/-/sitemaps/ accessible by the url /-/sitemaps. Since the top-level route - is already forbidden, we won't have any problem.