Remove extern crate tracing.

`use` is a nicer way of doing things.

compiler/rustc_data_structures/src/graph/implementation/mod.rs

@@ -22,6 +22,7 @@
 
 use rustc_index::bit_set::BitSet;
 use std::fmt::Debug;
+use tracing::debug;
 
 #[cfg(test)]
 mod tests;

compiler/rustc_data_structures/src/graph/implementation/tests.rs

@@ -1,4 +1,5 @@
 use crate::graph::implementation::*;
+use tracing::debug;
 
 type TestGraph = Graph<&'static str, &'static str>;
 

compiler/rustc_data_structures/src/graph/scc/mod.rs

@@ -10,6 +10,7 @@ use crate::graph::vec_graph::VecGraph;
 use crate::graph::{DirectedGraph, NumEdges, Successors};
 use rustc_index::{Idx, IndexSlice, IndexVec};
 use std::ops::Range;
+use tracing::{debug, instrument};
 
 #[cfg(test)]
 mod tests;