Become CPS compatible

[tpadioleau]

The Common Package Specification enforces the package name to match the namespace of the targets, see https://www.kitware.com/common-package-specification-is-out-the-gate

CPS addresses this by enforcing qualified target names, where the root “namespace” of the target is exactly the name of the package which provides that target. (This is also why existing CMake projects are strongly encouraged to use their package name as their CMake namespace when generating CMake-script exports, as this will reduce porting friction when transitioning to CPS.)

Do you mind if we change the name of the CMake project from "MDSpan" to "mdspan" ?