allow renv ignores to be set via option

[kevinushey]

Closes #863.

This PR allows one to set .renvignore-style ignores for renv via the option renv.renvignore.exclude. These can either be .gitignore-style ignores, or (if the asis attribute is set to TRUE) PCREs which match against the full file path. (Directories will have a trailing / appended to the path.)

The option is primarily intended for internal consumption, so I haven't documented any explicit interface here.

I think the usage for Packrat here would be something like:

# get ignores
ignores <- packrat::opts$ignored.directories()

# post-process ignore in some appropriate way

# tell renv to use the pattern as-is
attr(ignores, "asis", TRUE) <- TRUE

# set the option
options(renv.renvignore.exclude = ignores)

Let me know if this seems sane.

[aronatkins]

This change appears to work on my end.

Code like the following converts from the ignored.directories setting to an renv ignore pattern:

ignoresForRenv <- function(ignoredDirectories) {
  ignores <- NULL
  if (length(ignoredDirectories) > 0) {
    ignores <- ignoredDirectories
    # Make sure all the directories end with a slash.
    ignores <- ifelse(
        substr(ignores, nchar(ignores), nchar(ignores)) != "/",
        paste0(ignores, "/"),
        ignores
    )
    # Make sure all the directories begin with a slash.
    ignores <- ifelse(
        substr(ignores, 1, 1) != "/",
        paste0("/", ignores),
        ignores
    )
    attr(ignores, "asis") <- TRUE
    cat(paste0("renv ignore: ", ignores), sep = "\n")
  }
  ignores
}

That then feeds into an option-set:

  old_renv_exclude <- options("renv.renvignore.exclude" = ignoresForRenv(opts$ignored.directories()))
  on.exit(do.call(options, old_renv_exclude), add = TRUE)

One thing I had not done previously was forcing the same path and root for renv::dependencies; most of my previous experimentation left root = NULL (but I had been careful to setwd). With this set of changes, I have all tests passing.