[Dependency Scanning] On failure to locate a module, attempt to diagnose if binary dependencies contain search paths with this module.

Unlike with implicitly-built modules (prior to Swift 6 mode), explicitly-built modules require that all search paths be specified explicitly and no longer inherit search paths serialized into discovered Swift binary modules. This behavior was never intentional and is considered a bug. This change adds a diagnostic note to a scan failure: for each binary Swift module dependency, the scanner will attempt to execute a dependency scanning query for each serialized search path inside that module. If such diagnostic query returns a result, a diagnostic will be emitted to inform the user that the dependency may be found in the search path configuration of another Swift binary module dependency, specifying which search path contains the "missing" module, and stating that such search paths are not automatically inherited by the current compilation.

include/swift/DependencyScan/ModuleDependencyScanner.h

@@ -190,8 +190,24 @@ private:
   template <typename Function, typename... Args>
   auto withDependencyScanningWorker(Function &&F, Args &&...ArgList);
 
+  /// Use the scanner's ASTContext to construct an `Identifier`
+  /// for a given module name.
   Identifier getModuleImportIdentifier(StringRef moduleName);
 
+  /// Diagnose scanner failure and attempt to reconstruct the dependency
+  /// path from the main module to the missing dependency.
+  void diagnoseScannerFailure(const ScannerImportStatementInfo &moduleImport,
+                              const ModuleDependenciesCache &cache,
+                              std::optional<ModuleDependencyID> dependencyOf);
+
+  /// Assuming the \c `moduleImport` failed to resolve,
+  /// iterate over all binary Swift module dependencies with serialized
+  /// search paths and attempt to diagnose if the failed-to-resolve module
+  /// can be found on any of them.
+  void attemptToFindResolvingSerializedSearchPath(
+      const ScannerImportStatementInfo &moduleImport,
+      const ModuleDependenciesCache &cache, const SourceLoc &importLoc);
+
 private:
   const CompilerInvocation &ScanCompilerInvocation;
   ASTContext &ScanASTContext;