Allow CLI selection of a single rule to check.

[wxtim]

Closes #5544

Additionally...

Fix a bug where empty flow.cylc files caused traceback.

To replicate

mkdir ~/cylc-src/foo
touch ~/cylc-src/foo/flow.cylc
cylc lint ~/cylc-src/foo

Check List

• [x] I have read CONTRIBUTING.md and added my name as a Code Contributor.
• [x] Contains logically grouped changes (else tidy your branch by rebase).
• [x] Does not contain off-topic changes (use other PRs for other changes).
• [x] Applied any dependency changes to both setup.cfg (and conda-environment.yml if present).
• [x] Tests are included (or explain why tests are not needed).
• [x] CHANGES.md entry included if this is a change that can affect users
• [x] Cylc-Doc pull request opened if required at cylc/cylc-doc/pull/XXXX.
• [x] If this is a bug fix, PR should be raised against the relevant ?.?.x branch.

[oliver-sanders]

PR title and description don't appear to match?

Does this do both? If so maybe a bugfix changelog entry?

[wxtim]

Does this do both? If so maybe a bugfix changelog entry?

Yes. Sorry - original text not super clear. Have added the changelog entry as suggested.

[wxtim]

Redrafting - not a priority

[wxtim]

Roll this into the full refactor of Cylc Lint - probably a good training day activity for Tim