Move third-party crate root to a lib.rs file

Using /dev/null is no longer compatible with the most recent version of
Bazel's rules_rust crate_universe.

    INFO: Running command line: bazel-bin/third-party/vendor.sh
    Error: Package "third-party" target "third_party" had an absolute source path "/dev/null", which is not supported

Author: dtolnay

third-party/Cargo.toml

@@ -5,9 +5,6 @@ version = "0.0.0"
 edition = "2021"
 publish = false
 
-[lib]
-path = "/dev/null"
-
 [dependencies]
 cc = "1.0.83"
 clap = { version = "4", default-features = false, features = ["error-context", "help", "std", "usage"] }

third-party/src/lib.rs

@@ -0,0 +1 @@
+