Make triggers less susceptible to false positives

[wlandau]

Summary

cc @AlexAxthelm. In #478, we decided to detect dependencies from triggers: for example,

f <- function(){
  1 + 1
}
g <- function(){
  TRUE
}
plan <- drake_plan(
  x = target(
    command = f(),
    trigger = trigger(condition = g())
  )
)
vis_drake_graph(drake_config(plan))

g() may not be part of the command, but we should at least check and process it before we attempt x. The problem with this approach is that it is oversensitive to changes to g(). Clearly, if we change g() to function(){FALSE}, x should not build. But under #478, it does build because the depend trigger is still activated. And we don't want to use trigger(depend = FALSE) because we still want to react to changes to f().

The solution in this PR is to ignore changes to dependencies that only show up in the triggers. Implementation details:

• Store a new "ignore_changes" igraph attribute in build_drake_graph() to keep track of these trigger-only dependencies.
• In dependency_hash() and file_dependency_hash(), exclude everything listed in "ignore_changes".

The implementation admittedly added a little complexity to the code base, and it was not my most elegant work. However, it is consistent with the existing internals and design patterns.

Related GitHub issues and pull requests

• Ref: A new interface for triggers #473, An improved interface for triggers #478

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

Merging #480 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master   #480   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          67     67           
  Lines        5638   5666   +28     
=====================================
+ Hits         5638   5666   +28

Impacted Files	Coverage Δ
R/meta.R	100% <100%> (ø)	⬆️
R/config.R	100% <100%> (ø)	⬆️
R/graph.R	100% <100%> (ø)	⬆️
R/deprecate.R	100% <100%> (ø)	⬆️

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[AlexAxthelm]

I like it.