Specify all of msvcp$VER.dll, msvcr$VER.dll and vcruntime$VER.dll -- but check
each of them individually, because any given VS release has only a subset of
those. Add messaging to clarify what we're doing.

Introduce to_staging_dirs CMake macro to cut down on redundant boilerplate:
the idiom in which we use copy_if_different twice, once to the Release staging
directory and once to the RelWithDebInfo staging directory, each time
appending the target pathnames to third_party_targets. Replace that idiom with
calls to to_staging_dirs.