Before 19.03, programs.beets.enable implicitly followed whether
programs.beets.settings was non-empty. That compatibility branch was
still active for older state versions, but it was silent.
Route the default through the shared state-version helper so legacy
users get a consistent deprecation warning before the implicit
enablement is cleaned up. Add focused tests for the legacy and current
default branches alongside the existing beets coverage.
Signed-off-by: Austin Horstman <khaneliman12@gmail.com>
Instead of having to manually stub packages that should not be
downloaded we instead automatically stub all packages (except a small
list of whitelisted ones). Tests can re-introduce the real package by
using the `realPkgs` module argument.
