rollup merge of #23546: alexcrichton/hyphens

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/librustdoc/visit_ast.rs

@@ -237,7 +237,7 @@ impl<'a, 'tcx> RustdocVisitor<'a, 'tcx> {
             ast::ItemExternCrate(ref p) => {
                 let path = match *p {
                     None => None,
-                    Some((ref x, _)) => Some(x.to_string()),
+                    Some(x) => Some(x.to_string()),
                 };
                 om.extern_crates.push(ExternCrate {
                     name: name,