Guard Kokkos dependency

What is the problem

Finding Kokkos downloaded by FetchContent (e.g., in Ragnarok) causes trouble.

1. FetchContent libraries cannot be found using config mode or module mode find_package.
2. The install dependency might not find the correct Kokkos if the target is outside of the current project scope.

Solution

1. Kokkos target is checked before finding with the config mode.
2. Install rules are guarded when MemMan is used as a subproject.