Rollup merge of #31362 - jseyfried:fix_extern_crate_visibility, r=nikomatsakis

This PR changes the visibility of extern crate declarations to match that of items (fixes #26775).
To avoid breakage, the PR makes it a `public_in_private` lint to reexport a private extern crate, and it adds the lint `inaccessible_extern_crate` for uses of an inaccessible extern crate.

The lints can be avoided by making the appropriate `extern crate` declaration public.

diff --cc src/libsyntax/parse/parser.rs
Simple merge