Auto merge of #46550 - jseyfried:cleanup_builtin_hygiene, r=nrc

macros: hygienize use of `core`/`std` in builtin macros

Today, if a builtin macro wants to access an item from `core` or `std` (depending `#![no_std]`), it generates `::core::path::to::item` or `::std::path::to::item` respectively (c.f. `fn std_path()` in `libsyntax/ext/base.rs`).

This PR refactors the builtin macros to instead always emit `$crate::path::to::item` here. That is, the def site of builtin macros is taken to be in `extern crate core;` or `extern crate std;`. Since builtin macros are macros 1.0 (i.e. mostly unhygienic), changing the def site can only effect the resolution of `$crate`.

r? @nrc

src/librustc/hir/lowering.rs

@@ -175,7 +175,7 @@ pub fn lower_crate(sess: &Session,
     let _ignore = dep_graph.in_ignore();
 
     LoweringContext {
-        crate_root: std_inject::injected_crate_name(krate),
+        crate_root: std_inject::injected_crate_name(),
         sess,
         cstore,
         parent_def: None,