Find DeePMD-kit C/C++ library from CMake

After DeePMD-kit C/C++ library is installed, one can find DeePMD-kit from CMake:

find_package(DeePMD REQUIRED)

Note that you may need to add ${deepmd_root} to the cached CMake variable CMAKE_PREFIX_PATH.

To link against the C interface library, using

target_link_libraries(some_library PRIVATE DeePMD::deepmd_c)

To link against the C++ interface library, using

target_link_libraries(some_library PRIVATE DeePMD::deepmd_cc)