cli: infer run number where possible

[oliver-sanders]

Attempt after #3889, #4230

The default behaviour of cylc install will be to install workflows into an automatically named run (i.e. run<N>).

This means that the CLI will become a little cumbersome e.g.

# before
$ cylc stop my-flow
# after
$ cylc stop my-flow/run1

To combat this we should make the relevant Cylc subcommands (those with a REG argument) run-dir aware. That is to say that if the REG provided is not a workflow (denoted by the presence of a .service directory) but does contain automatically named runs (denoted by the presence of a runN symlink), then default to whatever run runN is pointing at.

This change returns the CLI REG argument to the way it was before for most cases whilst providing the ability to control named runs individually where required.

[MetRonnie]

Just to emphasise the last point (more of a note to self really): named runs (with cylc install foo --run-name=bar) should not be inferred (i.e. cylc play foo should not do anything, you would have to explicitly do cylc play foo/bar)

[MetRonnie]

Will attempt after #4180

[oliver-sanders]

Bumped the centralisation of workflow argument parsing into a separate issue to make this one a little clearer - #4230