Various types of R package dependencies

## Details

R packages may have various types of dependencies, see Writing R Extensions.pak groups dependencies into three groups:

• hard dependencies: "Depends", "Imports", and "LinkingTo",

• soft dependencies: "Suggests" and "Enhances",

• extra dependencies, see below.

pak supports concise ways of specifying which types of dependencies of a package should be installed. It is similar to how utils::install.packages() interprets its dependencies argument.You typically use one of these values:
• NA or "hard" to install a package and its required dependencies,

• TRUE to install all required dependencies, plus optional and development dependencies.

If you need more flexibility, the full description of possible values for the dependencies argument are:
• TRUE: This means all hard dependencies plus Suggests for direct installations, and hard dependencies only for dependent packages.

• FALSE: no dependencies are installed at all.

• NA (any atomic type, so NA_character_, etc. as well): only hard dependencies are installed.

• If a list with two entries named direct and indirect, it is taken as the requested dependency types, for direct installations and dependent packages.

• If a character vector, then it is taken as the dependency types for direct installations, and the hard dependencies are used for the dependent packages.

If "hard" is included in the value or a list element, then it is replaced by the hard dependency types. If "soft" or "all" is included, then it is replaced by all hard and soft dependency.

### Extra dependencies

pak supports extra dependency types for direct installations not from CRAN-like repositories. These are specified with a Config/Needs/ prefix in the DESCRIPTION and they can contain package references, separated by commas. For example you can specify packages that are only needed for the pkgdown website of the package:

Config/Needs/website: r-lib/pkgdown

To use these dependency types, you need to specify them in the dependencies argument to pak functions.

Note that Config/Needs/* fields are currently not used from CRAN packages, and packages in CRAN-like repositories in general.

Usually you specify that a Config/Needs/* dependency type should be installed together with "hard" or "all", to install all hard or soft dependencies as well.