FetchContent: Enforce FETCHCONTENT_FULLY_DISCONNECTED requirements

When FETCHCONTENT_FULLY_DISCONNECTED is set to true, FetchContent_MakeAvailable() and the single-argument form of FetchContent_Populate() require that the dependency's source directory has already been populated. CMake 3.29 and earlier did not check this requirement, but it is now enforced, subject to a new policy. This helps catch subtle, hard-to-trace problems where a dependency is not populated, but the project thinks it is and tries to use things that should be defined, but aren't.