cxxmodules: Do not expect collator install scripts with no CXX_MODULES

Calling `install(EXPORT)` with the `CXX_MODULES_DIRECTORY` parameter
leads to installation rules being generated which `include()` CMake
scripts that set the `IMPORTED_CXX_MODULES_[CONFIG]` target property for
relevant targets.  However, these scripts don't get generated for
targets in an export set which don't have any C++20 modules.  When the
installation rules attempt to `include()` the missing scripts, the
install fails.

Co-authored-by: Brad King <brad.king@kitware.com>

Source/cmExportBuildFileGenerator.cxx

@@ -542,6 +542,12 @@ bool cmExportBuildFileGenerator::GenerateImportCxxModuleConfigTargetInclusion(
   os.SetCopyIfDifferent(true);
 
   for (auto const* tgt : this->ExportedTargets) {
+    // Only targets with C++ module sources will have a
+    // collator-generated install script.
+    if (!tgt->HaveCxx20ModuleSources()) {
+      continue;
+    }
+
     os << "include(\"${CMAKE_CURRENT_LIST_DIR}/target-" << tgt->GetExportName()
        << '-' << config << ".cmake\")\n";
   }