Validate implicit task names

[oliver-sanders]

Built on #4343

Fixes small bug spotted whilst reviewing #4343 where it is possible to define root as an implicit task.

Also reserves _cylc.* in xtriggers and task names (used by internal xtriggers).

Testing checklist:

• Implicit task names in graph section (only) caught by validate/play.
• Implicit task names in runtime section (only) caught by validate/play.
• Retry xtriggers still work ok.
• Auto generated docs for the TaskNameValidator make sense.

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.py and
  conda-environment.yml.
• Appropriate tests are included (unit and/or functional).
• Appropriate change log entry included.
• No documentation update required (auto documented from TaskNameValidator object)

[hjoliver]

@oliver-sanders - note I pushed a trivial test fix to your branch, to see if all tests will pass here now.

[hjoliver]

... they do 🎉 ... any reason this is still a Draft? Change log entry?

[oliver-sanders]

Waiting for me to get around to rebasing, branch got in a mess.

[wxtim]

Should this:

[scheduler]
    allow implicit tasks = true

[scheduling]
    initial cycle point = 1066

    [[dependencies]]
        P1Y = root

[runtime]
    [[foo]]

validate?

I don't think it should, but it does for me.

[oliver-sanders]

Good catch, it turns out that the graph parser was expanding root to a list of tasks, but only if there were tasks defined in the runtime section.

Fixed and extended the test to cover this.