Support semi-absolute offset triggers?

[hjoliver]

[cylc]
  UTC mode = True
[scheduling]
  initial cycle point = 20150108T00
  initial cycle point constraints = T00
  final cycle point =   20150109T12
  [[dependencies]]
    [[[P1D]]]
       graph = foo
    [[[PT12H]]]
       graph = foo[T00] => bar
[runtime]
  [[foo, bar]]

The intention is that foo runs on a daily cycle, and bar runs 12-hourly triggering off of the most recent foo.

I wasn't aware that we supported this, but surprisingly (to me!) it almost works at Cylc 7 (even with offsets like [T00-P1D], except that the T00 doesn't evaluate to "the day of the task's cycle point with T00 appended" but it's offset from that to create one past and one future trigger:

2021-07-22T00:48:18Z INFO - [bar.20150108T0000Z] -triggered off ['foo.20150108T0000Z']
2021-07-22T00:48:20Z INFO - [bar.20150108T1200Z] -triggered off ['foo.20150109T0000Z']
2021-07-22T00:48:22Z INFO - [bar.20150109T0000Z] -triggered off ['foo.20150109T0000Z']
2021-07-22T00:48:23Z INFO - [bar.20150109T1200Z] -holding (future trigger beyond stop point)

Using Cylc 8, we only get bar triggering on the daily cycle.