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/tasks.rs

@@ -1,2 +1,3 @@
 pub mod clippy;
 pub mod licenses;
+pub mod package_conformity;