rustc: Add support for extern crate foo as bar

The compiler will now issue a warning for crates that have syntax of the form
`extern crate "foo" as bar`, but it will still continue to accept this syntax.
Additionally, the string `foo-bar` will match the crate name `foo_bar` to assist
in the transition period as well.

This patch will land hopefully in tandem with a Cargo patch that will start
translating all crate names to have underscores instead of hyphens.

cc #23533

src/libsyntax/std_inject.rs

@@ -54,8 +54,8 @@ impl fold::Folder for StandardLibraryInjector {
 
         // The name to use in `extern crate "name" as std;`
         let actual_crate_name = match self.alt_std_name {
-            Some(ref s) => token::intern_and_get_ident(&s[..]),
-            None => token::intern_and_get_ident("std"),
+            Some(ref s) => token::intern(&s),
+            None => token::intern("std"),
         };
 
         krate.module.items.insert(0, P(ast::Item {
@@ -64,7 +64,7 @@ impl fold::Folder for StandardLibraryInjector {
             attrs: vec!(
                 attr::mk_attr_outer(attr::mk_attr_id(), attr::mk_word_item(
                         InternedString::new("macro_use")))),
-            node: ast::ItemExternCrate(Some((actual_crate_name, ast::CookedStr))),
+            node: ast::ItemExternCrate(Some(actual_crate_name)),
             vis: ast::Inherited,
             span: DUMMY_SP
         }));