Missing exported functions pkg_*

[llrs-roche]

I was reading ?pkg_ref and realized that there are several functions documented but not exported (and only can be used with riskmetric:::pkg_*:

pkg_install(x, lib.loc = NULL)
pkg_source(x)
pkg_cran(x, repos = getOption("repos", "https://cran.rstudio.com"))
pkg_bioc(x)
pkg_missing(x)
pkg_library(lib.loc)

Maybe they could be exported or avoid documenting them as it is a bit confusing for a new user. I can put together a PR to export them.