We are currently loading plugin commands stubs for every
command invocation to add support for Cobra v2 completion.
This cause a significant performance hit if there is a
lot of plugins in the user space (7 atm in Docker Desktop):
`docker --version` takes in current 23.0.1 ~93ms
Instead of removing completion for plugins to fix the
regression, we can slightly improve plugins discovery by
spawning a goroutine for each iteration in the loop when
listing plugins:
`docker --version` now takes ~38ms
Signed-off-by: CrazyMax <crazy-max@users.noreply.github.com>