c++modules: Avoid adding file to multiple file sets in synthetic targets

The introduction of file set type `SOURCES` can raise error on a
duplicate file due to erroneous synthetic targets definition.

Source/cmGeneratorTarget.cxx

@@ -5433,7 +5433,8 @@ bool cmGeneratorTarget::DiscoverSyntheticTargets(
 
       // Copy file sets.
       {
-        for (auto const* gfs : gt->GetAllFileSets()) {
+        for (auto const* gfs :
+             gt->GetInterfaceFileSets(cm::FileSetMetadata::CXX_MODULES)) {
           auto* newFs =
             tgt
               ->GetOrCreateFileSet(gfs->GetName(), gfs->GetType(),

Tests/RunCMake/CXXModulesCompile/RunCMakeTest.cmake

@@ -164,6 +164,8 @@ if ("named" IN_LIST CMake_TEST_MODULE_COMPILATION)
   run_cxx_module_test(file-sets-with-dot)
   run_cxx_module_test(vs-without-flags)
   run_cxx_module_test(library library-static -DBUILD_SHARED_LIBS=OFF)
+  run_cxx_module_test(library library-static-file-set-SOURCES -DBUILD_SHARED_LIBS=OFF
+                                                              -DWITH_FILE_SET_SOURCES=ON)
   run_cxx_module_test(unity-build)
   run_cxx_module_test(object-library)
   run_cxx_module_test(generated)

Tests/RunCMake/CXXModulesCompile/library/CMakeLists.txt

@@ -23,6 +23,13 @@ target_sources(library
     importable.cxx)
 target_compile_features(library PUBLIC cxx_std_20)
 
+if(WITH_FILE_SET_SOURCES)
+  target_sources(library
+    PUBLIC
+      FILE_SET SOURCES
+      FILES src.cpp)
+endif()
+
 add_executable(exe)
 target_link_libraries(exe PRIVATE library)
 target_sources(exe

Tests/RunCMake/CXXModulesCompile/library/src.cpp

@@ -0,0 +1,3 @@
+void src()
+{
+}