This patch implements the plan described in https://internals.rust-lang.org/t/privacy-and-its-interaction-with-docs-lints-and-stability/2880 with one deviation.
It turns out, that rustdoc needs the "directly public" set for its docs inlining logic, so the privacy pass have to produce three sets and not two. Three is arguably too many, so I merged them in one map:
`public_items/exported_items/reachable_items: NodeSet => access_levels: NodeMap<AccessLevel>`
r? @alexcrichton
6a6b97cf02