Returns (and creates on request) a per-user cache directory under
tools::R_user_dir(). A cache directory serves one plan, so give each plan
a directory of its own inside this one, as in
file.path(scopus_cache_dir(), "my-plan"), and pass that as cache_dir to
scopus_fetch_plan(). The cache is entirely optional, and
scopus_cache_clear() removes it, every plan's directory included.