a logical indicating whether dependencies of pkg and their
dependencies (and so on) should be included.
Can also be a character vector listing the types of dependencies, a subset
of c("Depends", "Imports", "LinkingTo", "Suggests", "Enhances").
Character string "all" is shorthand for that vector, character string
"most" for the same vector without "Enhances", character string "strong"
(default) for the first three elements of that vector.