contrib.url adds the appropriate type-specific path within a
repository to each URL in repos.contrib.url(repos, type = getOption("pkgType"))install.packages.
repos.type = "both" this will use the source repository.setRepositories to set options("repos"),
the most common value used for argument repos. available.packages, download.packages,
install.packages. The ‘R Installation and Administration’ manual for how to
set up a repository.