CLI ui simplification

[puerco]

This PR implements the UI simplification initially proposed in #366

All of the original commands have been preserved hidden but the new CLI control surface looks like this:

Usage:
  sourcetool [command]

Verification Commands:
  audit          Verifies multiple commits in the branch history
  get            Fetch and print the attestations for a revision
  status         Check the SLSA Source status of a repo/branch
  verify         Verifies the specified commit is valid

Attestation Commands:
  attest         Generate the source attestations for a revision

Policy Commands:
  policy         tools to work with source policies

Configuration & Setup Commands:
  auth           Manage user authentication
  setup          configure SLSA source features in a repository

Additional Commands:
  completion     Generate the autocompletion script for the specified shell
  help           Help about any command

In addition to the plan outlined in the issue, we now have a new --from=[github,notes] flag that controls where sourcetool reads the repository attestations.

Closes #366