Fix output usage for flux get <sources|images>

[stealthybox]

Fixes #2346

Background:

Cobra will automatically print Usage for commands that do not have a .RunE.
All of the flux get command tree (including the flux get root) have --watch/-w
registered as a PersistentFlag.
The validateWatchOption() func is used to short circuit runnable sub-commands that
operate on multiple kinds, since the watch is not designed to stream/output multiple GVK's.

Justification:

These commands are runnable and need filtering:

• flux get all
• flux get sources all
• flux get images all

These commands are not runnable, so there should be no RunE at all:

• flux get sources
• flux get images

Current Bug/Inconsistency:

$ flux get sources
# outputs nothing

$ flux get sources -w
✗ expected a single resource type, but found sources

$ flux get images
# outputs nothing

$ flux get images -w
✗ expected a single resource type, but found images

however:

$ flux get
# outputs Usage

$ flux get -w
# still outputs Usage

Result of this patch:

$ flux get sources
# outputs Usage

$ flux get sources -w
# outputs Usage

$ flux get images
# outputs Usage

$ flux get images -w
# outputs Usage

ps:
I miss you nerds
much love from the tanzu team

[kingdonb]

Thanks for explaining @stealthybox this is very helpful information, I can confirm this addresses the issue after running it locally 👍 we nerds miss you as well!

[stefanprodan]

LGTM

Thanks @stealthybox 🏅