Fatal Error HtmlRouteProvider not found?
Unless you’re doing something custom, your classes need to be under ./src to be discovered. By default, MODULE/src/Foo.php is mapped to the Drupal\MODULE\Foo namespace and so on.

Move the two classes out of the root of your module and into ./src, clear cache, and try again.

See PSR4 namespaces and autoloading in Drupal 8 for full details.