Use api-builder for defplan and plan-fn
Clean up pallet.plan requires
Move execute-target-phase to pallet.plan
Move target function to pallet.spec

Moves target-with-specs, target-plan, target-phase, and phase-spec to pallet.spec
from pallet.phase.
Add pallet.user/default-user

Expose the default admin user via a function, rather than just
via *admin-user*.