Restrict scope of dependency detection

[wlandau]

Summary

In build_drake_graph(), global dependencies are now restricted to objects in the environment and targets in the plan. The implementation relies on a new globals argument to code_dependencies(), which lists all the allowable globals.

Related GitHub issues and pull requests

• Ref: More consistent dependency detection #519

Checklist

• I have read drake's code of conduct, and I agree to follow its rules.
• I have read the guidelines for contributing.
• 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

❗ No coverage uploaded for pull request base (master@41fba6b). Click here to learn what that means.
The diff coverage is 100%.

@@           Coverage Diff           @@
##             master   #522   +/-   ##
=======================================
  Coverage          ?   100%           
=======================================
  Files             ?     71           
  Lines             ?   6391           
  Branches          ?      0           
=======================================
  Hits              ?   6391           
  Misses            ?      0           
  Partials          ?      0

Impacted Files	Coverage Δ
R/build_drake_graph.R	100% <100%> (ø)
R/envir.R	100% <100%> (ø)
R/dependencies.R	100% <100%> (ø)

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