cran_version() should be more robust

[ateucher]

Several functions (use_release_checklist(), use_news_md()) call cran_version() to check if a package is on CRAN, which fails if a CRAN mirror is not set.

cran_version() calls available.packages() which in turn calls contrib.url() with repos = getOption("repos"). If the CRAN mirror is unset (the default value is "@CRAN@" in a factory fresh installation) it will prompt a user to choose in an interactive session, or fail otherwise:

withr::with_options(
  list(repos = c(CRAN = "@CRAN@")), 
  usethis:::cran_version()
)
#> Error in contrib.url(repos, type): trying to use CRAN without setting a mirror

This is showing up in revdep checks and CRAN checks for some usethis reverse depenencies which use use_news_md(). E.g., cynkra/fledge#683

cran_version() should not fail if there is no internet, or if there is no CRAN mirror set. In the former case, it should likely return NULL, which it currently returns if the package is not on CRAN. That seems like a reasonable fallback. If there is no CRAN mirror set we should try to set one locally, and also wrap that in a tryCatch(), returning NULL again if still unsuccessful.

This was surfaced in revdepchecks for #1849, was at first thought to be a false positive, and did not cause issues on incoming checks, but is causing failures for revdeps on CRAN.

[ateucher]

If getOption(repos)["CRAN"] is NULL or NA (I don't know under what circumstances it would be set this way), cran_version() currently returns NULL. By locally setting a CRAN mirror we should be able to have success in more of these cases as well.

[krlmlr]

Are you aiming for a point release anytime soon? Thanks!

[jennybc]

I don't have concrete plans because this is literally the only thing that would be in it. Since we just released 1 year's worth of maintenance, I suspect a few more smallish things will come up in the medium term. So maybe once there's a couple little things, we'll do a patch release.

[jennybc]

I ended up needing to do a patch release anyway, so this fix will presumably be in usethis 2.2.1.