Changes to Pathauto

I've reviewed the current state of the pathauto module. 
As I'm new to D8 development, I did some minor changes I saw during the reading of the code.
The reason that was moved to a protected method is to be able to easily mock it in unit tests if we would add those.

We could possibly instead inject the entity manager in the constructor now and get the storage from that, then just do $this->termStorage->loadTree(). That is however not important, we want to remove the term specific logic from the pathauto manager anyway, so it is temporary.