auto merge of #17134 : vberger/rust/lint_unused_extern_crate, r=alexcrichton

This PR creates a new lint : ``unused_extern_crate``, which do pretty much the same thing as ``unused_import``, but for ``extern crate`` statements. It is related to feature request #10385.

I adapted the code tracking used imports so that it tracks extern crates usage as well. This was mainly trial and error and while I believe all cases are covered, there might be some code I added that is useless (long compile times didn't give me the opportunity to check this in detail).

Also, I removed some unused ``extern crate`` statements from the libs, that where spotted by this new lint.

src/test/compile-fail/lint-unused-extern-crate.rs

@@ -0,0 +1,32 @@
+// Copyright 2014 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+#![feature(globs)]
+#![deny(unused_extern_crate)]
+#![allow(unused_variable)]
+
+extern crate libc; //~ ERROR: unused extern crate
+
+extern crate "collections" as collecs; // no error, it is used
+
+extern crate rand; // no error, the use marks it as used
+                   // even if imported objects aren't used
+
+extern crate time; // no error, the use * marks it as used
+
+#[allow(unused_imports)]
+use rand::isaac::IsaacRng;
+
+use time::*;
+
+fn main() {
+    let x: collecs::vec::Vec<uint> = Vec::new();
+    let y = now();
+}