Engine is a workspace (pantsbuild#5555)

Now that we're using a cdylib, this can be enabled.

This allows rust-standard ways of operating across a whole project, like
running `cargo test --all`.

It also improves compile time. Using a workspace means that a single
`Cargo.lock` file is used for the project, and a single `target`
directory is used for outputs. This in turn means that if building
multiple components (e.g. `engine` and `fs_util`), the intermediate
artifacts are actually shared across components, rather than
re-compiled.

build-support/bin/ci.sh

@@ -220,7 +220,10 @@ fi
 if [[ "${skip_rust_tests:-false}" == "false" ]]; then
   start_travis_section "RustTests" "Running Pants rust tests"
   (
-    "${REPO_ROOT}/src/rust/engine/run-all-tests.sh"
+    source "${REPO_ROOT}/build-support/pants_venv"
+    source "${REPO_ROOT}/build-support/bin/native/bootstrap.sh"
+    activate_pants_venv
+    RUST_BACKTRACE=1 PANTS_SRCPATH="${REPO_ROOT}/src/python" ensure_cffi_sources=1 run_cargo test "${MODE_FLAG}" --all --manifest-path="${REPO_ROOT}/src/rust/engine/Cargo.toml"
   ) || die "Pants rust test failure"
   end_travis_section
 fi

build-support/bin/release.sh

@@ -236,7 +236,7 @@ function build_pants_packages() {
     RUST_BACKTRACE=1 PANTS_SRCPATH="${ROOT}/src/python" run_cargo build --release --manifest-path="${ROOT}/src/rust/engine/fs/fs_util/Cargo.toml"
     dst_dir="${DEPLOY_DIR}/bin/fs_util/$("${ROOT}/build-support/bin/get_os.sh")/${version}"
     mkdir -p "${dst_dir}"
-    cp "${ROOT}/src/rust/engine/fs/fs_util/target/release/fs_util" "${dst_dir}/"
+    cp "${ROOT}/src/rust/engine/target/release/fs_util" "${dst_dir}/"
   ) || die "Failed to build fs_util"
   end_travis_section