Need policy on modules packaging in cmake.

[JeffGarland]

See this review https://github.com/bemanproject/scope/pull/49/files#r2598630577

also disucssion here: https://discourse.bemanproject.org/t/c-20-modules-in-beman-project-libraries/279/31

For packaging modules we need to understand if/where the bmi is to be placed in the release tree. Not sure what folks are doing w.r.t. conan, vcpkg, etc.

[ClausKlein]

see too https://github.com/ClausKlein/execution/blob/feature/prepare-cxx_module-tests/docs/cxx_modules_cheatsheeet.md