742: Extern crate r=matklad a=flodiebold
This implements `extern crate` declarations by lowering them to (absolute) imports, and adds support for absolute paths. It also extracts the extern prelude from the per-module item map, and handles the special case of extern crates in the crate root adding to the extern prelude.
This means we finally resolve `Arc`, so it fixes#523😄
Co-authored-by: Florian Diebold <flodiebold@gmail.com>