Vendor libtest's dependencies in the rust-src component

This is the Rust side of rust-lang/wg-cargo-std-aware#23

Author: Gankra

src/bootstrap/dist.rs

@@ -1040,6 +1040,25 @@ impl Step for Src {
             builder.copy(&builder.src.join(file), &dst_src.join(file));
         }
 
+        // libtest includes std and everything else, so vendoring it
+        // creates exactly what's needed for `cargo -Zbuild-std` or any
+        // other analysis of the stdlib's source. Cargo also needs help
+        // finding the lock, so we copy it to libtest temporarily.
+        //
+        // Note that this requires std to only have one version of each
+        // crate. e.g. two versions of getopts won't be patchable.
+        let dst_libtest = dst_src.join("library/test");
+        let dst_vendor = dst_src.join("vendor");
+        let root_lock = dst_src.join("Cargo.lock");
+        let temp_lock = dst_libtest.join("Cargo.lock");
+        builder.copy(&root_lock, &temp_lock);
+
+        let mut cmd = Command::new(&builder.initial_cargo);
+        cmd.arg("vendor").arg(dst_vendor).current_dir(&dst_libtest);
+        builder.run(&mut cmd);
+
+        builder.remove(&temp_lock);
+
         // Create source tarball in rust-installer format
         let mut cmd = rust_installer(builder);
         cmd.arg("generate")