Centralised reg parse should handle "./" better

[MetRonnie]

Follow-up to #4285

The centralised reg parse should recognise that ./<reg> means only look in cwd for <reg>, not in ~/cylc-run. But with #4285 you get an unnecessary warning:

$ cd ~/cylc-src
$ cylc validate ./armageddon 
WARNING - The specified reg could refer to ./armageddon/flow.cylc or ~/cylc-run/armageddon/run1/flow.cylc.
This command will use ./armageddon/flow.cylc.

[MetRonnie]

Also, (and note this is slightly different problem), if the cwd is ~/cylc-run/, then sometimes it still gives the warning message despite the fact that ./darmok/flow.cylc and ~/cylc-run/darmok/flow.cylc are the same file in that case. This may be due to our Met Office filesystem where /net/home/ and /home/ are the same.

[MetRonnie]

This can probably be bumped to the 8.0rc1 milestone, or even 8.0.0