VignetteBuilder: field of DESCRIPTION should declare all builder deps

[infotroph]

From "Writing R Extensions" (emphasis theirs):

The ‘VignetteBuilder’ field names (in a comma-separated list) packages that provide an engine for building vignettes. These may include the current package, or ones listed in ‘Depends’, ‘Suggests’ or ‘Imports’. The utils package is always implicitly appended. See Section 1.4.2 [Non-Sweave vignettes], page 48, for details. Note that if, for example, a vignette has engine ‘knitr::rmarkdown’, then knitr (https://CRAN.R-project.org/package=knitr) provides the engine but both knitr and rmarkdown (https://CRAN.R-project.org/package=rmarkdown) are needed for using it, so both these packages need to be in the ‘VignetteBuilder’ field and at least suggested (as rmarkdown is only suggested by knitr, and hence not available automatically along with it). Many packages using knitr (https://CRAN.R-project.org/package=knitr) also need the package formatR (https://CRAN.R-project.org/package=formatR) which it suggests and so the user package needs to do so too and include this in ‘VignetteBuilder’.

Since all our vignettes use knitr::rmarkdown, my takeaway is that we need to replace VignetteBuilder: knitr with VignetteBuilder: knitr, rmarkdown in all packages that have vignettes.