Fix ignore-modules breakage.

This resolves the failing test, with the cause being that
expandModuleSets (in ModuleResolver.pm) did *not* handle ignored
modules, only resolveSelectorsIntoModules did.

ksb::Application's modulesFromSelectors calls one or the other based
on whether the user had command line arguments to select a specific
subset of modules or not.

This is fixed by moving the check to expandModuleSets (which is called
from resolveSelectorsIntoModules), which also required permitting
a function to expand a single module set to expand to an empty list (if
all modules were ignored).