Allow Symlinking for log/job

[wxtim]

closes #6145

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/748.
• [x] If this is a bug fix, PR should be raised against the relevant ?.?.x branch.

[wxtim]

cylc clean needs to be updated to reflect this change.

Mildly irked that there isn't a single source of truth about the symlink dirs, but I ended up with circular import issues trying to do that.

Also worth trying this out with remote platforms to make sure clean works in that context too.

Yes - have done.

[oliver-sanders]

Mildly irked that there isn't a single source of truth about the symlink dirs

There was before!

Try moving your new dict in with the previous one. Circular import issues can be solved.

[oliver-sanders]

Worth looking at testing cylc clean with log/job symlink dirs setup.

I would have thought tests/functional/cylc-clean/00-basic.t would have noticed the dirs left behind by cylc clean, but it didn't seem to?