Send less data to remote transient workers

[wlandau]

In make(parallelism = "future") (future.R) each transient worker only needs the imports and the direct target dependencies. The other targets do not need to be sent.

[kendonB]

Doesn't future handle this automatically for elements in the global environment? And for other items, shouldn't the workers just read from the .drake folder themselves?

[wlandau]

Yes, future uses the globals package to detect elements of the global environment, but drake is using its own dependency detection technique, and the results may disagree.

[wlandau]

And because of NFS file latency issues, I would rather not have transient workers read from the .drake folder before building their targets. I think we can find a win-win here.

[wlandau]

Like many things, this will be much easier after #440.

[wlandau]

On second thought, the environment is pruned before the globals are chosen, so any affected users can just choose make(pruning_strategy = "memory").