Expand comma separated lists in global.cylc:[install][symlink dirs]

[wxtim]

From an issue raised by @dpmatthews

Allow comma separated lists of install targets in global.cylc:[install][symlink dirs][__MANY__]

@oliver-sanders tagged for code review, @dpmatthews for concept review.

Note

Slightly cheekily, I've targeted 3.8.x not master. Although this is a feature, it's for sysadmins not end-users, and I have very high confidence that it won't break anything.

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 if present).
• Tests are included (or explain why tests are not needed).
• Changelog entry included if this is a change that can affect users
• Cylc-Doc pull request opened if required at cylc/cylc-doc/pull/XXXX.
• If this is a bug fix, PR should be raised against the relevant ?.?.x branch.