Introduce `--workspace`

[MaulingMonkey]

Checklist

• [x] I have read the Contributor Guide
• [x] I have read and agree to the Code of Conduct
• [x] I have added a description of my changes and why I'd like them included in the section below

Description of Changes

This adds a new command line flag, --workspace, allowing you to enumerate the licenses used by an entire workspace, even in the presence of a package inside the root of the workspace.

Related Issues

Closes https://github.com/EmbarkStudios/cargo-about/issues/151 which rambles on about reasons/alternatives

[MaulingMonkey]

cargo-deny error is unrelated, and fixed by #149