Remove superfluous parallel backends

[wlandau]

Summary

• Remove all backends accept "clustermq", "future", "hasty", and "future_lapply_staged".
• @kendonB, "future_lapply_staged" parallelism is our secret. It's still there, but it is not officially supported or even in the documentation. I still intend to remove it ASAP, but probably not for 7.0.0. I simplified it down to 68 lines of code, which is not too terrible in the short term (R/future_lapply_staged.R).
• Build all the imports before all the targets. This greatly simplifies the internals.
• The parallelism and jobs arguments to make() are now officially scalars (cc @bpbond). Use the jobs_preprocess argument to parallelize the imports and other elements of the preprocessing.
• Use automatic staged parallelism for the imports, based on parLapply() on Windows and mclapply() on more programmer-friendly systems.

Related GitHub issues and pull requests

• Ref: Remove all non-clustermq parallel backends? #561 (not totally done yet)

Checklist

• I have read drake's code of conduct, and I agree to follow its rules.
• I have listed any substantial changes in the development news.
• I have added testthat unit tests to tests/testthat to confirm that any new features or functionality work correctly.
• I have tested this pull request locally with devtools::check()
• This pull request is ready for review.
• I think this pull request is ready to merge.

[codecov-io]

Codecov Report

Merging #648 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master   #648    +/-   ##
======================================
  Coverage     100%   100%            
======================================
  Files          80     73     -7     
  Lines        6223   5611   -612     
======================================
- Hits         6223   5611   -612

Impacted Files	Coverage Δ
R/triggers.R	100% <ø> (ø)	⬆️
R/priority_queue.R	100% <ø> (ø)	⬆️
R/test_scenarios.R	100% <ø> (ø)	⬆️
R/handlers.R	100% <ø> (ø)	⬆️
R/utils.R	100% <ø> (ø)	⬆️
R/drake_debug.R	100% <ø> (ø)	⬆️
R/imports.R	100% <100%> (ø)
R/build_times.R	100% <100%> (ø)	⬆️
R/check.R	100% <100%> (ø)	⬆️
R/deprecate.R	100% <100%> (ø)	⬆️
... and 15 more

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[bpbond]

Seems very in keeping with move to lighter, tighter, faster package.