Restrict sysroot crate imports to those defined in this repo.

It's common to import dependencies from the sysroot via `extern crate`
rather than use an explicit cargo dependency, when it's necessary to use
the same dependency version as used by rustc itself. However, this is
dangerous for crates.io crates, since rustc may not pull in the
dependency on some targets, or may pull in multiple versions. In both
cases, the `extern crate` fails to resolve.

To address this, re-export all such dependencies from the appropriate
`rustc_*` crates, and use this alias from crates which would otherwise
need to use `extern crate`.

compiler/rustc_codegen_gcc/src/gcc_util.rs

@@ -1,8 +1,8 @@
 #[cfg(feature = "master")]
 use gccjit::Context;
 use rustc_codegen_ssa::target_features;
+use rustc_data_structures::smallvec::{SmallVec, smallvec};
 use rustc_session::Session;
-use smallvec::{SmallVec, smallvec};
 
 fn gcc_features_by_flags(sess: &Session, features: &mut Vec<String>) {
     target_features::retpoline_features_by_flags(sess, features);