don't suggest feature(rustc_private) for std dependencies

[jyn514]

Code

// https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=00a9b664d929655758d33fc1469ee274
extern crate compiler_builtins;

Current output

error[E0658]: use of unstable library feature 'rustc_private': this crate is being loaded from the sysroot, an unstable location; did you mean to load this crate from crates.io via `Cargo.toml` instead?
 --> src/lib.rs:2:1
  |
2 | extern crate compiler_builtins;
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: see issue #27812 <https://github.com/rust-lang/rust/issues/27812> for more information
  = help: add `#![feature(rustc_private)]` to the crate attributes to enable

Desired output

error[E0463]: can't find crate for `compiler_builtins`
 --> src/lib.rs:2:1
  |
2 | extern crate compiler_builtins;
  | ^^^^^^^^^^^^^^^^^ can't find crate

Rationale and extra context

Dependencies of the standard library are an implementation details and shouldn't affect diagnostics. In particular, I saw a case in the wild where someone added extern crate libc; and got the false impression that libc was nightly-only; that could have been avoided with better diagnostics.

Note that I think we should still giving this message for crates whose name starts with rustc_ - those only show up when you have rustc-dev installed already, and the hint may be useful.

Other cases

No response

Anything else?

No response

[jyn514]

duplicate of #50373 (comment)