Test renamed crates in rustdoc

Author: kornelski

tests/rustdoc/extern/extern-html-alias.rs

@@ -0,0 +1,8 @@
+//@ compile-flags:-Z unstable-options --extern-html-root-url extern_prelude_name=https://renamed.example.com  --extern-html-root-url empty=https://bad.invalid
+//@ aux-crate:extern_prelude_name=empty.rs
+
+extern crate extern_prelude_name as internal_ident_name;
+
+//@ has extern_html_alias/index.html
+//@ has - '//a/@href' 'https://renamed.example.com/empty/index.html'
+pub use internal_ident_name as yet_different_name;