Show how targets were expanded with wildcards

[wlandau]

Summary

1. Implement a trace argument to evaluate_plan() to optionally add indicator columns to show which targets got expanded/evaluated with which wildcard values.
2. Rename the always_rename argument to rename in evaluate_plan().
3. Add a rename argument to expand_plan().
4. Clean up the internals of wildcard templating.

(1) solves #376 because it allows easier custom filtering for later gathering. @tmastny, I know this is not quite what you asked for, but it makes custom gathering easier while preserving the flexibility and design patterns of drake.

library(drake)
plan <- drake_plan(x = rnorm(1e3, mean = custom__), y = rt(1e3, df = 5))
eval <- evaluate_plan(plan, wildcard = "custom__", values = 1:2, trace = TRUE)
gather_plan(eval[!is.na(eval$custom__), ], gather = "c")
#> # A tibble: 1 x 2
#>   target command                
#>   <chr>  <chr>                  
#> 1 target c(x_1 = x_1, x_2 = x_2)

(1) also opens up new visualization possibilities because it provides natural groupings for targets.

Related GitHub issues

• Ref: Wildcard alternative to gather/reduce_plan #376

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

@@          Coverage Diff          @@
##           master   #461   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          66     66           
  Lines        5475   5485   +10     
=====================================
+ Hits         5475   5485   +10

Impacted Files	Coverage Δ
R/check.R	100% <100%> (ø)	⬆️
R/sanitize.R	100% <100%> (ø)	⬆️
R/generate.R	100% <100%> (ø)	⬆️

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[lintr-bot]

tests/testthat/test-generate.R:402:1: style: Trailing whitespace is superfluous.

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