message: Update FindRAJA.cmake to use the exported RAJA CMake configs
diff: # - RAJA_LIBRARIES
  # - RAJA_INCLUDE_DIRS
  -include(MfemCmakeUtilities)
  -mfem_find_package(RAJA RAJA RAJA_DIR "include" "RAJA/RAJA.hpp" "lib" "RAJA"
  - "Paths to headers required by RAJA." "Libraries required by RAJA.")
  -
  -if (NOT RAJA_CONFIG_CMAKE)
  - set(RAJA_CONFIG_CMAKE "${RAJA_DIR}/share/raja/cmake/raja-config.cmake")
  +if (RAJA_FOUND)
  + return()
  +endif()
  +message(STATUS "Looking for RAJA ...")
  +if (RAJA_DIR)
  + message(STATUS " in RAJA_DIR = ${RAJA_DIR}")
  + # As a workaround for an issue in raja-config.cmake we try to load
  + # bltTargets.cmake before calling find_package(RAJA ...).
  + if (NOT RAJA_BLT_TARGETS)
  + set(RAJA_BLT_TARGETS "${RAJA_DIR}/lib/cmake/raja/bltTargets.cmake")
  + endif()
  + if (EXISTS "${RAJA_BLT_TARGETS}")
  + include("${RAJA_BLT_TARGETS}")
  + endif()
  + find_package(RAJA CONFIG NO_DEFAULT_PATH PATHS "${RAJA_DIR}")
  +endif()
  +if (NOT RAJA_FOUND)
  + message(STATUS " in standard CMake locations")
  + find_package(RAJA CONFIG)
  endif()
  -if (EXISTS "${RAJA_CONFIG_CMAKE}")
  - include("${RAJA_CONFIG_CMAKE}")
  - if (ENABLE_CUDA AND NOT MFEM_USE_CUDA)
  - message(FATAL_ERROR
  - "RAJA is built with CUDA: MFEM_USE_CUDA=YES is required")
  +if (RAJA_FOUND)
  + set(RAJA_LIBRARIES "RAJA" CACHE STRING "RAJA imported target." FORCE)
  + set(RAJA_INCLUDE_DIRS "" CACHE STRING "RAJA include dirs (not used)" FORCE)
  + message(STATUS
  + "Found RAJA target: ${RAJA_LIBRARIES} (version: ${RAJA_VERSION})")
  +else()
  + set(msg STATUS)
  + if (RAJA_FIND_REQUIRED)
  + set(msg FATAL_ERROR)
  endif()
  + message(${msg} "RAJA not found. Please set RAJA_DIR to the RAJA prefix.")
  endif()
  