explicitSelection method
Resolves the explicit selection, or null to signal "ask interactively".
Returned keys are the optional module keys (core is always added later).
Implementation
List<String>? explicitSelection() {
if (minimal) return const [];
if (all) return kModules.keys.toList();
if (modules.isNotEmpty) return modules;
if (yes) return kModules.keys.toList();
return null;
}