Change sbctl repo to use new package names

[xavpaice]

Per the design in replicatedhq/troubleshoot#893, the proposed package names to avoid conflicts with Troubleshoot should be implemented prior to moving the sbctl repo to Troubleshoot.

Additionally, create a new sbutil module to store reusable support bundle functionality. At least Troubleshoot's analyze & sbctl's serve modules will depend on this module once the repos are combined.

Package Structure

cmd
    serve/
    shell/
pkg
    sbutil/
    serve/
    shell/
tests           # API tests, integration tests etc reside here.
    serve/      # sbctl BDD tests.
        # all tests and test fixtures moved here

Definition of done

• In the sbctl repo, the api, k8s, sbctl, util packages are adjusted/removed/renamed to avoid conflicts with the packages in the Troubleshoot repo
• new packages for sbutil, serve and shell have been created in the sbctl repo itself
• new commands for serve and shell have been created, either in Troubleshoot by importing sbctl, or in the sbctl repo itself