Allowing/denying rustc internal lints silently fails

[camelid]

I would think it'd give a feature-gate error.

#![deny(rustc::default_hash_types)]

type Foo = std::collections::HashSet<()>;

   Compiling playground v0.0.1 (/playground)
warning: type alias is never used: `Foo`
 --> src/lib.rs:3:1
  |
3 | type Foo = std::collections::HashSet<()>;
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: `#[warn(dead_code)]` on by default

warning: `playground` (lib) generated 1 warning
    Finished dev [unoptimized + debuginfo] target(s) in 2.21s

[jyn514]

@camelid these are tool lints, the same way clippy:: and rustdoc:: are tool lints. I think it's reasonable to remove rustc from known_tool_names so you have to opt-in with register_tool; but that only seems worth it if we can stop special-casing rustc:: lints to only run when -Zunstable-options is passed - I think @oli-obk ran into some trouble with that in the past.