Add custom file_out() files back to dependency graph visuals

[wlandau]

Examples like #451 (comment) will be much easier to visualize this way. And from #483, detailed input/output file dependency information is available to drake_graph_info().

[wlandau]

An example modified from #451 should be a good test case:

library(drake)
library(glue)
library(rlang)
library(tidyverse)

write_command <- function(cmd, inputs = NULL , outputs = NULL){
  inputs <- enexpr(inputs)
  outputs <- enexpr(outputs)
  expr({
    glue_data(
      list(
        inputs = file_in(!!inputs),
        outputs = file_out(!!outputs)
      ),
      !!cmd
    ) %>%
      lapply(FUN = system)
  }) %>%
    expr_text
}

meta_plan <- tribble(
  ~cmd, ~inputs, ~outputs,
  "cat {inputs} > {outputs}", c("in1.txt", "in2.txt"), c("out1.txt", "out2.txt"),
  "cat {inputs} {inputs} > {outputs}", c("out1.txt", "out2.txt"), c("out3.txt", "out4.txt")
)

plan <- tibble(
  target = paste0("target_", seq_len(nrow(meta_plan))),
  command = pmap_chr(meta_plan, write_command)
) %>%
  print

writeLines("in2", "in1.txt")
writeLines("in2", "in2.txt")
make(plan)
writeLines("out3", "out3.txt")

config <- drake_config(plan)
vis_drake_graph(config)