make_pgk_cache_subdir_for_relative_file: Given a relative file, create the subdir in the package cache...
In pkgfilecache: Download and Manage Optional Package Data

Given a relative file, create the subdir in the package cache if needed.

1	make_pgk_cache_subdir_for_relative_file(pkg_info, relative_file)

`pkg_info,`	named list. Package identifier, see get_pkg_info() on how to get one.
`relative_file,`	string or vector of strings. If a string, this function does nothing. If a vector of strings, a path is created from the elements using file.path, and the directory of it (determined by dirname()) is created.