xtask: Convert workspacing checks to new lint structure

[smalis-msft]

Now it doesn't have to traverse the filesystem itself or run cargo metadata, which saves us about half a second.

[Copilot]

Pull request overview

Moves the “verify workspace Cargo.toml workspacing” checks from a standalone xtask pass into the shared lint framework, avoiding filesystem traversal and cargo metadata for faster runs.

Changes:

• Removed the VerifyWorkspace fmt pass and its filesystem/cargo metadata implementation.
• Added a new workspaced lint that validates workspace membership/dependency/exclusion and checks for non-workspaced dependencies.
• Updated lint error reporting to log using self.path rather than workspace_dir.join(self.path).

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 7 comments.

File	Description
xtask/src/tasks/fmt/workspace.rs	Removes the old workspace verification task implementation (walk FS + cargo metadata).
xtask/src/tasks/fmt/mod.rs	Drops the VerifyWorkspace fmt pass and module wiring.
xtask/src/tasks/fmt/lints/workspaced.rs	Introduces a new lint implementing workspace/member/dependency checks and dependency workspacing validation.
xtask/src/tasks/fmt/lints.rs	Registers the new lint and adjusts error logging to use self.path.