Rollup merge of #37524 - alexcrichton:vendor, r=brson

Vendor all rustbuild dependencies in this repo

This commit vendors all crates.io dependencies into the rust-lang/rust repository using the `cargo-vendor` tool. This is done in an effort to make rustbuild distro-ready by ensuring that our source tarballs are self-contained units which don't need extraneous network downloads.

A new `src/vendor` directory is created with all vendored crates, and Cargo, when using rustbuild, is configured to use this directory. Over time we can deduplicate this directory with the actual src tree (e.g. src/librustc_serialize, src/liblibc, src/libgetopts, ...). For now though that's left to a separate commit.

diff --cc src/bootstrap/bootstrap.py
Simple merge