[ObjC] Add ModuleForFile helper method in ImportWriter (#12012)

Custom `protoc` plugins may use this method to look up the framework name for a given file if they want to generate code not related to `import`s.

Closes #12012

COPYBARA_INTEGRATE_REVIEW=https://github.com/protocolbuffers/protobuf/pull/12012 from dnkoutso:import_writer_module_for_file 57844831d5
PiperOrigin-RevId: 512629671

src/google/protobuf/compiler/objectivec/import_writer.cc

@@ -140,15 +140,11 @@ void ImportWriter::AddFile(const FileDescriptor* file,
     return;
   }
 
-  // Lazy parse any mappings.
-  if (need_to_parse_mapping_file_) {
-    ParseFrameworkMappings();
-  }
+  auto module_name = ModuleForFile(file);
 
-  auto proto_lookup = proto_file_to_framework_name_.find(file->name());
-  if (proto_lookup != proto_file_to_framework_name_.end()) {
+  if (!module_name.empty()) {
     other_framework_imports_.emplace_back(absl::StrCat(
-        proto_lookup->second, "/", FilePathBasename(file), header_extension));
+        module_name, "/", FilePathBasename(file), header_extension));
     return;
   }
 
@@ -166,6 +162,23 @@ void ImportWriter::AddRuntimeImport(const std::string& header_name) {
   protobuf_imports_.push_back(header_name);
 }
 
+std::string ImportWriter::ModuleForFile(const FileDescriptor* file) {
+  ABSL_DCHECK(!IsProtobufLibraryBundledProtoFile(file));
+
+  // Lazy parse any mappings.
+  if (need_to_parse_mapping_file_) {
+    ParseFrameworkMappings();
+  }
+
+  auto proto_lookup = proto_file_to_framework_name_.find(file->name());
+
+  if (proto_lookup != proto_file_to_framework_name_.end()) {
+    return proto_lookup->second;
+  }
+
+  return "";
+}
+
 void ImportWriter::PrintFileImports(io::Printer* p) const {
   if (!other_framework_imports_.empty()) {
     for (const auto& header : other_framework_imports_) {

src/google/protobuf/compiler/objectivec/import_writer.h

@@ -55,6 +55,9 @@ class ImportWriter {
 
   void AddFile(const FileDescriptor* file, const std::string& header_extension);
   void AddRuntimeImport(const std::string& header_name);
+  // This can return an empty string if there is no module for the file. It also
+  // does not handle bundled proto files.
+  std::string ModuleForFile(const FileDescriptor* file);
 
   void PrintFileImports(io::Printer* p) const;
   void PrintRuntimeImports(io::Printer* p, bool default_cpp_symbol) const;