Add standard way to load the right Distributed{,Next} package

As discussed with @oschulz, as more folks adopt DistributedNext it might be good to have a lightweight package like MPIPreferences.jl to standardize loading the right Distributed* library.

Start by reviewing DistributedNext.jl and the referenced MPIPreferences.jl approach. The issue names no files, tests, or entry points, so first determine how Distributed and DistributedNext are currently loaded and what a standard package should cover. Done means agreeing on and implementing a lightweight, documented way to load the appropriate Distributed* library.

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