This idea follows from the "Suggests" section of the R-pkgs book, where you provide the code:

if (!requireNamespace("pkg", quietly = TRUE)) {
  stop("Package \"pkg\" needed for this function to work. Please install it.",
    call. = FALSE)
}

Could this be useful as a new assertion,has_package()?

assertthat::has_package("dplyr")

I'd be happy to make the PR if you think this might be useful. If so, some questions:

1. Name: has_package(), has_packages(), other?
2. Should the argument be ... as unnamed, or packages which would be a character-vector of package names, or other?