Before moving on with the Controller we set out to write, let's break down the namespace situation in Drupal 8 and how the folder structure needs to be inside a module.
Drupal 8 uses the PSR-4 namespace autoloading standard. In effect, this means that the namespace of all Drupal core and module classes starts with \Drupal. For modules, the base namespace is \Drupal\module_name, where module_name is the machine name of the module. This then maps to the /src folder found inside the module directory (for main integration files). For PHPUnit tests, we have a different namespace, as we will see later in the book.
So essentially, we will need a /src folder inside our module to place all of our classes that need to be autoloaded. So, we can go ahead and create it.