With this change, the Controller is now aware of its own listeners in a
more codified and structured way.
The reasoning behind these changes are as follows:
1. Currently, we're iterating over these listeners in multiple spots
along this initialization flow to check each listeners' purpose. By
doing this once when we create a Controller and essentially storing the
result, we can clean up the downstream code to use those results.
2. Initial effort to centralize Controller validation. At the moment, we
are doing a lot of validation all over the `cmd` and `config` packages.
From a conceptual perspective, Controller-related validation should
be performed in the function that is responsible for constructing the
Controller object itself. This enables any developer to look in a
single place for all the constraints that we place on creating a
Controller.
Hugo Vieira
8d48a8515c