xtask: Add command for checking packages conform to certain standards (#15236)

This PR adds a new `xtask` command for checking that packages conform to
certain standards.

Still a work-in-progress, but right now it checks:

- If `[lints] workspace = true` is set
- If packages are using non-workspace dependencies

Release Notes:

- N/A

tooling/xtask/src/workspace.rs

@@ -1,17 +1,9 @@
-use std::fs;
-
-use anyhow::{anyhow, Result};
-use cargo_toml::{Manifest, Workspace};
-use toml;
+use anyhow::{Context, Result};
+use cargo_metadata::{Metadata, MetadataCommand};
 
 /// Returns the Cargo workspace.
-pub fn load_workspace() -> Result<Workspace> {
-    let workspace_cargo_toml = fs::read_to_string("Cargo.toml")?;
-    let workspace_cargo_toml: Manifest = toml::from_str(&workspace_cargo_toml)?;
-
-    let workspace = workspace_cargo_toml
-        .workspace
-        .ok_or_else(|| anyhow!("top-level Cargo.toml is not a Cargo workspace"))?;
-
-    Ok(workspace)
+pub fn load_workspace() -> Result<Metadata> {
+    MetadataCommand::new()
+        .exec()
+        .context("failed to load cargo metadata")
 }