C++ 20 Modules Update

[ben.boeckel (Ben Boeckel (Kitware))]

@Zingam

target_sources("${PROJECT_NAME}"
PUBLIC
  FILE_SET cxx_modules TYPE CXX_MODULES
  FILES
    "src/MetaInfo/Details.cpp"
    "src/MetaInfo/Details.cppm"

The .cpp file is not a module file (i.e., it does not produce a BMI). It is instead an implementation unit of a module; it does not belong in CXX_MODULES filesets.