- The architecture of letting someone build up a list of requirements
from data emitted by the ModuleManager is broken: if it gets loaded
later, it will miss data; passing around complicated objects is
no fun anyway. Get rid of it, on the way to "ModuleManager has
its own model of requirements".
- Give the ModuleManager a RequirementsModel -- that is the source
of truth about the module-requirements of the modules managed
by that particular ModuleManager.
- Let the RequirementsChecker operate on a given RequirementsModel.
- the checker only collects and calls requirements; it has no
UI component, and only manages data (and a thread to do the
checking). Move it out of the UI library.
