remove rustc-serialize dependency and factor util::cargo out into a crate

clippy_lints/src/lib.rs

@@ -38,8 +38,6 @@ extern crate regex_syntax;
 // for finding minimal boolean expressions
 extern crate quine_mc_cluskey;
 
-extern crate rustc_serialize;
-
 extern crate rustc_errors;
 extern crate rustc_plugin;
 extern crate rustc_const_eval;