[OTHER]: Check if robin_map is already included

[GMellar]

I use the robin map in my project as a submodule and set the variable NB_USE_SUBMODULE_DEPS to false. However the check for this submodule uses CMakeFindDependencyMacro which does find it. I think it would be better to omit this check completely and just link to tsl::robin_map when NB_USE_SUBMODULE_DEPS is true so the user of nanobind can decide to use the submodule.

Another approach which I use sometimes is to check if the target is already defined and do not include any subdirectory when this is the case.

[wjakob]

I don't quite understand the issue. Are you not saying that CMakeFindDependencyMacro finds the right dependency? Or did I misunderstand?

If a change of this part of the CMake logic is needed, could I ask you to make a concrete proposal by opening a PR?