Adjust to SourceType::InTree in several places

These were left over in migrations to subtrees, which should generally be treated
as-if it was local.

Also fixes a warning caused by this change.

Author: Mark-Simulacrum

src/bin/cg_clif_build_sysroot.rs

@@ -8,7 +8,6 @@
 
 #![feature(rustc_private)]
 
-extern crate rustc_data_structures;
 extern crate rustc_driver;
 extern crate rustc_interface;
 extern crate rustc_session;