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recent changes mean users can select backend at run time. make backends
opt-out in spackage.

This reverts commit 8b4060bb.
- no longer needed; upstream Mercury has the ability to set this with
  runtime hooks now

building from scratch on this loaner laptop caught a few missing bits

Phil says "we're probably going to have a bad time without it; if we end
up in some permutations where it doesn't work well then we probably
ought to face up and fix it".   Sounds good to me.

rpmem requires a fairly new libfabirc and a patch to make gcc7 happy

David's added a lot of features. Features need more mochi packages.

Useful sometimes for performance analysis

Updated mercury to provide a "self-forward" variant.  Updated mobject to
depend on this verison plus a few other packages it now needs.

libfabric seems pretty good these days.  Can always opt-in to CCI if
platform needs that.