Fix parentless spawning.

[hjoliver]

Close #7235
Close #5730

See #7235 (comment)

The correct way to do this is: for any task that is parentless in one or more recurrences, always spawn the next parentless instance (which may lie beyond multiple parented instances) out to the first parentless instance beyond the runahead limit.

In other words, we should treat the parentless instances of sometimes-parentless tasks the same way we treat always-parentless tasks, leaving parented instances to be spawned by their parents.

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.

[hjoliver]

DRAFT: this change breaks a bunch of tests because it messes with the way sequential xtriggered tasks are managed, so I have to work through that (hopefully that's the only problem).