CMake should expose the same target regardless of FASTGLTF_ENABLE_CPP_MODULES

[n0F4x]

Right now, fastgltf::module is exported alongside fastgltf::fastgltf, so a user could link to both. However, this is undesirable. A build should only contain one or the other.
I propose exposing the same target for both cases. (No more fastgltf::module)