update rustc-std-workspace crates

- make rustc-std-workspace-core/alloc re-exports of their underlying crates, like std
= cleanup manifests

Author: RalfJung

rustc-std-workspace-core/README.md

@@ -27,3 +27,6 @@ it'll look like
 
 when Cargo invokes the compiler, satisfying the implicit `extern crate core`
 directive injected by the compiler.
+
+The sources for the crates.io version can be found in
+[`src/rustc-std-workspace`](../../src/rustc-std-workspace).