Disallow nested run directories

[hjoliver]

Reported by @kinow

Currently you can register and run nested flows:

$ cylc register f1 ~/suites/foo
$ cylc register f1/f2 ~/suites/foo

Cylc scan (and hence the UI) does not see the running f1/f2 flow though, because it stops recursing deeper into cylc-run/ whenever it finds a valid run directory.

Can't think of a good reason to allow this - particularly given this cylc scan problem and the potential for huge run directories.

We should prevent this at the registration stage.

[MetRonnie]

Should we always prevent f1/f2 or only if f1 is valid run directory ( - and what is valid? If it has .service or flow.cylc/suite.rc in it?)

cc @oliver-sanders

[oliver-sanders]

The .service directory is created in the registration process so would be the easiest option.

[oliver-sanders]

Couple of things:

1. In-place installations (e.g. run a suite in a temp dir)
   • Should we permit these, e.g. in the work dir (for subsuites).
2. Might be a good time to remove the cylc register --run-dir=RUNDIR option.
   • Obsolete once we have symlink support built-in.