outdated - Show diff of packages for outdated / upgrade --noop to review changes

As a user I would like to be able to find out what changed in packages that are about to be updated to find out if there were changes that might be malicious, e.g. download url is suspicious for the package or the install script performs other steps than previously.

Acceptance tests:
- The `outdated` command shows a hint to view the diff of the packages files that are outdated
- The `upgrade --noop` command shows a hint to view the diff of the packages files that are outdated
- There is a switch for the `outdated` and `upgrade --noop` command that creates a diff of the packages files that are outdated/about to be upgraded

Start with the entry points for the `outdated` and `upgrade --noop` commands, then trace how they identify package files and render command output. Done means both commands provide a diff switch and show a hint for outdated or soon-to-be-upgraded package files, as specified by the acceptance tests.

Written by the indexing model from the issue text.