Use http_archive for buck crates without vendoring

[dtolnay]

This eliminates the cargo vendor and reindeer step. Users can checkout the repo and immediately run buck2 build.

$ git clone https://github.com/dtolnay/cxx
$ cd cxx
$ git submodule update --init
$ cargo vendor --versioned-dirs --locked --manifest-path third-party/Cargo.toml third-party/vendor
$ reindeer --third-party-dir third-party buckify
$ buck2 build ...

[dtolnay]

@steveklabnik — I had to make some changes to the prelude's implementation of http_archive and rust_library, so if you are interested in this you'll need an updated prelude along with new reindeer.