allow knitr targets to depend on target aliases

[fmichonneau]

Maybe I'm not thinking about this correctly, but It seems that it would be useful to support dependencies "aliases" in the knitr targets to avoid repetition. See processed_data in the example below.

sources:
  - code.R

targets:
  all:
    depends: plot.pdf

  processed_data:
     depends:
        - processed_data_1
        - processed_data_2
        - processed_data_3

  data.csv:
    command: download_data(target_name)

  processed_data_1:
    command: process_data("data1.csv")

  processed_data_2:
    command: process_data("data2.csv")

  processed_data_3:
    command: processed_data_alt("data1.csv")

  plot.pdf:
    command: myplot(processed)
    plot: true

  report.md:
    depends:
      - processed_data
    knitr: true

[richfitz]

Good idea! There's a question of how deep that should go (aliases that include only other aliases perhaps?). But the simplest case would be "a target that does not have a command" and then stop there.