[Dependency Scanning] Deprecate/Remove batch scanning capability

Batch dependency scanning was added as a mechanism to support multiple compilation contexts within a single module dependency graph.
The Swift compiler and the Explicitly-built modules model has long since abandoned this approach and this code has long been stale. It is time to remove it and its associated C API.

lib/DependencyScan/ModuleDependencyScanner.cpp

@@ -78,7 +78,6 @@ findPathToDependency(ModuleDependencyID dependency,
                      const ModuleDependenciesCache &cache) {
   auto mainModuleDep = cache.findDependency(cache.getMainModuleName(),
                                             ModuleDependencyKind::SwiftSource);
-  // We may be in a batch scan instance which does not have this dependency
   if (!mainModuleDep.has_value())
     return {};
   auto mainModuleID = ModuleDependencyID{cache.getMainModuleName().str(),