Add functions for core packages

[ErdaradunGaztea]

The best bet is probably to use installed.packages(priority = "base"). The implementations would be as follows:

wood_core_packages <- function() {
  rownames(installed.packages(priority = "base"))
}

wood_core_version <- function(package) {
  installed.packages(priority = "base")[package, "Version"]
}

wood_core_dependencies <- function(package) {
  # Some function that extracts dependencies here
  extract_dependencies(installed.packages(priority = "base")[package, ])
}

Plus some argument checks, docs, and tests; obviously.