Get rid of cpp_modules scripts

[alandefreitas]

The problem

The lib doesn't have any non-dev dependencies. Catch2 is only used for tests. There's no point in the complexity of the cpp_modules.cmake script.

Proposed solution

Remove the cpp_modules.cmake script and just use find_package/FetchContent for the tests, as in this example.

Alternatives I've considered

Maintaining things as they are.

Additional context

cpp_modules.cmake is there for historical reasons only.