Auto merge of #96085 - jsgf:deny-unused-deps, r=compiler-errors

Auto merge of #96085 - jsgf:deny-unused-deps, r=compiler-errors

Make sure `-Dunused-crate-dependencies --json unused-externs` makes rustc exit with error status

This PR:
- fixes compiletest to understand unused extern notifications
- adds tests for `--json unused-externs`
- makes sure that deny-level unused externs notifications are treated as compile errors
  - refactors the `emit_unused_externs` callstack to plumb through the level as an enum as a string, and adds `Level::is_error`

Update: adds `--json unused-externs-silent` with the original behaviour since Cargo needs it. Should address `@est31's` concerns.

Fixes: https://github.com/rust-lang/rust/issues/96068