Better scan workflow detection.

[hjoliver]

Attempted do-over of scan workflow detection, after noticing that an old workflow definition (i.e. suite.rc) installed by Cylc 8 is invisible (to cylc scan) until you run it.

New logic based on new explanatory comments in the code, which I've verified by manual testing.

This is a small change with no associated Issue.

Requirements check-list

• I have read CONTRIBUTING.md and added my name as a Code Contributor.
• Contains logically grouped changes (else tidy your branch by rebase).
• Does not contain off-topic changes (use other PRs for other changes).
• Applied any dependency changes to both setup.cfg and conda-environment.yml.

• Appropriate tests are included (unit and/or functional).

• Appropriate change log entry included.

• No documentation update required.

[oliver-sanders]

Cheers.