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>