We move the implementations of our secrets managers in to
`pkg/secrets` (which is where the base64 one lives) and wire their use
up during deserialization.
It's a little unfortunate that for the passphrase based secrets
manager, we have to require `PULUMI_CONFIG_PASSPHRASE` when
constructing it from state, but we can make more progress with the
changes as they are now, and I think we can come up with some ways to
mitigate this problem a bit (at least make it only a problem for cases
where you are trying to take a stack reference to another stack that
is managed with local encryption).
