[meta] add a workspace-hack package managed by hakari (#4154)

Similar to oxidecomputer/crucible#956, this adds
a workspace-hack package managed by cargo-hakari. This should massively
cut down on incremental build times.

`cargo hakari` doesn't yet support writing out `workspace-hack`
dependency lines using workspace inheritance
(guppy-rs/guppy#7), so skip over it in the
lint.

Reverting this change can be done by following the instructions at https://docs.rs/cargo-hakari/latest/cargo_hakari/#disabling-and-uninstalling.

.config/hakari.toml

@@ -0,0 +1,29 @@
+# This file contains settings for `cargo hakari`.
+# See https://docs.rs/cargo-hakari/latest/cargo_hakari/config for a full list of options.
+
+hakari-package = "omicron-workspace-hack"
+
+# Format for `workspace-hack = ...` lines in other Cargo.tomls. Requires cargo-hakari 0.9.8 or above.
+dep-format-version = "4"
+
+# Setting workspace.resolver = "2" in the root Cargo.toml is HIGHLY recommended.
+# Hakari works much better with the new feature resolver.
+# For more about the new feature resolver, see:
+# https://blog.rust-lang.org/2021/03/25/Rust-1.51.0.html#cargos-new-feature-resolver
+resolver = "2"
+
+# Add triples corresponding to platforms commonly used by developers here.
+# https://doc.rust-lang.org/rustc/platform-support.html
+platforms = [
+    "x86_64-unknown-linux-gnu",
+    "x86_64-apple-darwin",
+    "aarch64-apple-darwin",
+    "x86_64-unknown-illumos",
+    # "x86_64-pc-windows-msvc",
+]
+
+[traversal-excludes]
+workspace-members = ["xtask"]
+
+# Write out exact versions rather than a semver range. (Defaults to false.)
+# exact-versions = true

.github/workflows/hakari.yml

@@ -0,0 +1,37 @@
+# This workflow file serves as an example for cargo-hakari CI integration.
+
+on:
+  push:
+    branches:
+      - main
+  pull_request:
+    branches:
+      - main
+
+name: cargo hakari
+
+jobs:
+  workspace-hack-check:
+    name: Check workspace-hack
+    runs-on: ubuntu-latest
+    env:
+      RUSTFLAGS: -D warnings
+    steps:
+      - uses: actions/checkout@v4
+      - uses: actions-rs/toolchain@v1
+        with:
+          toolchain: stable
+      - name: Install cargo-hakari
+        uses: taiki-e/install-action@v2
+        with:
+          tool: cargo-hakari
+      - name: Check workspace-hack Cargo.toml is up-to-date
+        uses: actions-rs/cargo@v1
+        with:
+          command: hakari
+          args: generate --diff
+      - name: Check all crates depend on workspace-hack
+        uses: actions-rs/cargo@v1
+        with:
+          command: hakari
+          args: manage-deps --dry-run