Set implicit R dependency explicitly

[MichaelChirico]

I am trying to use {rcmdcheck} for our "ancient" tests on R 3.2.0+, but it doesn't work because some of {rcmdcheck}'s upstream dependencies themselves now depend on higher versions of R:

Dependency	R dependency
callr	3.4
cli	3.4
desc	3.4
pkgbuild	3.5
with	3.5
proccesx (via xopen)	3.4

Obviously all the better if those upstream dependencies could allow an even older version of R to be tested in CI, but at least declaring {rcmdcheck} Depends: R (>= 3.5.0) would make for easier failure messaging.

[gaborcsardi]

Right now it is R >= 3.6 because of R6 and withr, but I am not sure if we should do this. It is unlikely that we'd be able to keep this current in the future, and if we dropped some dependencies in the future, then rcmdcheck would have an R version requirement for nothing.

Also, rcmdcheck probably works with earlier versions of those packages, but if we add the R version requirement here, that's not the case any more.

FWIW pak's messaging is reasonably clear that it currently needs R 3.6:

> pak::pkg_install("r-lib/rcmdcheck")
✔ Updated metadata database: 4.27 MB in 12 files.
ℹ R 3.5 x86_64-apple-darwin15.6.0 packages are missing from CRAN
✔ Updating metadata database ... done
Error:
! error in pak subprocess
Caused by error:
! Could not solve package dependencies:
* r-lib/rcmdcheck:
  * Can't install dependency R6
  * Can't install dependency withr
* R6: Needs R >= 3.6
* withr: Needs R >= 3.6.0