Merge #742

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>

crates/ra_lsp_server/src/project_model/sysroot.rs

@@ -127,6 +127,7 @@ rustc_tsan
 syntax";
 
 const STD_DEPS: &str = "
+alloc
 alloc_jemalloc
 alloc_system
 core