swarming: use subcommands.Section
swarming: use subcommands.Section

This shows help like
```
Client tool to access a swarming server.

Usage:  swarming [command] [arguments]

Commands:

        task related commands

  cancel        cancel a task
  collect       Waits on a set of Swarming tasks
  reproduce     reproduces a task locally
  request-show  returns properties of a request
  spawn-tasks   Spawns a set of Swarming tasks
  tasks         lists tasks
  trigger       Triggers a Swarming task

        bot related commands

  bots          lists bots
  delete-bots   delete bots
  terminate     terminate a bot

        other commands

  help          prints help about a command
  whoami        prints an email address associated with currently cached token
  login         performs interactive login flow
  logout        removes cached credentials
  version       prints the executable version


Use "swarming help [command]" for more information about a command.
Use "swarming help -advanced" to display all commands.
```

Change-Id: I6f91384cc2772ae9a8ff8555016a627b4334d06e
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/3247471
Auto-Submit: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>
Commit-Queue: Junji Watanabe <aaef61ef98ffea505db831f310be2a0eb8492537@google.com>
Reviewed-by: Junji Watanabe <aaef61ef98ffea505db831f310be2a0eb8492537@google.com>

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