support use of mdspan/mdarray.hpp header

[cwsmith]

Using #include "mdspan/mdarray.hpp" provided by mdspan fails with the following error:

In file included from /space/cwsmith/mdspan/buildMdsCuda/install/include/mdspan/mdarray.hpp:29,
                 from /space/cwsmith/mdspan/mdarrayReproducer/mdarrayReproducer.cpp:17:         
/space/cwsmith/mdspan/buildMdsCuda/install/include/mdspan/../experimental/__p1684_bits/mdarray.hpp:19:10: fatal error: ../mdspan: No such file or directory
   19 | #include "../mdspan"                                                                                                            
      |          ^~~~~~~~~~~                                                                                                            
compilation terminated. 

A reproducer (mdarrayReproducer.cpp) is below:

#include "mdspan/mdarray.hpp"
int main(int argc, char* argv[]) {
  return 0;
}

and its CMakeLists.txt:

cmake_minimum_required(VERSION 3.16)
project(mdarrayReproducer CXX)
find_package(mdspan REQUIRED)
add_executable(reproducer mdarrayReproducer.cpp)
target_link_libraries(reproducer std::mdspan)

This PR fixes the compilation error, but I'm not sure if it is the correct fix. If it is merged and then brought into downstream Kokkos, it will resolve the issue discussed in kokkos/kokkos#7671.

All mdspan tests passed when using the following cmake commands with GCC 12.3.0 and CUDA 12.1.1.

bdir=$PWD/buildMdsCuda
cmake -S mdspan -B $bdir \
  -DCMAKE_INSTALL_PREFIX=$bdir/install \
  -DMDSPAN_CXX_STANDARD=17 \
  -DMDSPAN_ENABLE_TESTS=on \
  -DMDSPAN_ENABLE_EXAMPLES=on \
  -DMDSPAN_ENABLE_CUDA=on \
  -DCMAKE_CUDA_FLAGS=--expt-relaxed-constexpr \
  -DCMAKE_CUDA_ARCHITECTURES=86 
cmake --build buildMdsCuda -j20 --target install
ctest --test-dir buildMdsCuda/

[nmm0]

I think this would work with just "../mdspan/mdspan.hpp".

The issue here is we switched the main headers to mdspan.hpp instead of mdspan, and I guess we missed this. I guess this only failed in Kokkos because we don't install the legacy mdspan header

[cwsmith]

#include "../../mdspan/mdspan.hpp" worked. Should I push that change?

[nmm0]

Yes I think that would be good thank you!

[cwsmith]

9582f5b has the change