add support for FILE_SET CXX_MODULES

Since `CMake v3.30` this is possible and should be supported too:

```cmake
install(TARGETS Also
      EXPORT Algo
      FILE_SET CXX_MODULES
          DESTINATION ${CMAKE_INSTALL_LIBDIR}/cmake/my_package/src
      CXX_MODULES_BMI
          DESTINATION ${CMAKE_INSTALL_LIBDIR}/cmake/my_package/bmi-$<CONFIG>
      ...  # Other things to install
  )
```