[Caching] Support CrossImport modules for caching build

Add support for cross import modules by ingesting swiftoverlay files for
the cross import into CAS file system.

The long-term better fix will be just passing the cross import
information from scanner to swift-frontend so frontend doesn't need to
read overlay files again to figure out the cross import module.

rdar://123839248

include/swift/DependencyScan/ModuleDependencyScanner.h

@@ -72,9 +72,8 @@ public:
                           DiagnosticEngine &diags, bool ParallelScan);
 
   /// Identify the scanner invocation's main module's dependencies
-  llvm::ErrorOr<ModuleDependencyInfo> getMainModuleDependencyInfo(
-      ModuleDecl *mainModule,
-      std::optional<SwiftDependencyTracker> tracker = std::nullopt);
+  llvm::ErrorOr<ModuleDependencyInfo>
+  getMainModuleDependencyInfo(ModuleDecl *mainModule);
 
   /// Resolve module dependencies of the given module, computing a full
   /// transitive closure dependency graph.