Update find_package for msgpack to work with 5.x and 6.x.

[stellaraccident]

Adapted from: https://github.com/msgpack/msgpack-c/wiki/Q%26A#how-to-support-both-msgpack-c-c-version-5x-and-6x-

Note that 6.x also defines compile definitions that must be respected.

[davidd-amd]

@stellaraccident How/why where tests added to this PR? Also, we will need to get @KKyang or @jichangjichang to approve as well.

[stellaraccident]

No longer needed with next-cmake