Sub folders and how to namespace classes In Drupal 7 I can have a module inside another module. Is this still the case in Drupal 8 and how would I namespace classes inside the sub module?

Another thing I can do in Drupal 7 is to create sub folders inside the modules folder to group my modules.  

Example:

sites
--- / all
--- / --- / modules
--- / --- / --- / Collection_Alpha
--- / --- / --- / --- / module_a
--- / --- / --- / --- / module_b
--- / --- / --- / Collection_Beta
--- / --- / --- / --- / module_x
--- / --- / --- / --- / module_y  

Is this still possible in Drupal 8? If yes, how would I have to namespace my classes so the autoloader can resolve it to the correct folder structure?

e.g.: Drupal\Collection_Alpha\module_a\myClass1 ?
@Clive is correct.

The namespace is always Drupal\{module_name}. Where that module lives is irrelevant (as long as it is somewhere where Drupal can find it). That namespace always points to $path_to_module/src.

It would not possibly work in any other way. Different sites have modules in a different structure and different places, but the same code has to work no matter where.