Finds the listeners of a signal module.

This works like protocol consolidation. A resource that listens to signals defines a module named Ash.Signals.Listeners.<Resource>, which returns its listeners. Listeners are found by looking for modules with that prefix, among the loaded modules and the .beam files on the code path. Neither the signal module nor any domain needs to know about them.

The :ash_signals compiler does this once per build, and compiles the result into an Ash.Signals.Dispatch.<SignalModule> module, so emitting a signal does not search for listeners. When there is no dispatch module, because the compiler is not used or ash: [consolidate_signals: false] is set in the project, listeners are searched for each time a signal is emitted. See Mix.Tasks.Compile.AshSignals.