allow option-ext as a tool dependency (MPL licensed)

Author: RalfJung

src/tools/tidy/src/deps.rs

@@ -47,6 +47,7 @@ const EXCEPTIONS: &[(&str, &str)] = &[
     ("instant", "BSD-3-Clause"),                             // rustc_driver/tracing-subscriber/parking_lot
     ("mdbook", "MPL-2.0"),                                   // mdbook
     ("openssl", "Apache-2.0"),                               // opt-dist
+    ("option-ext", "MPL-2.0"),                               // cargo-miri (via `directories`)
     ("rustc_apfloat", "Apache-2.0 WITH LLVM-exception"),     // rustc (license is the same as LLVM uses)
     ("ryu", "Apache-2.0 OR BSL-1.0"),                        // cargo/... (because of serde)
     ("self_cell", "Apache-2.0"),                             // rustc (fluent translations)