Rename deploy.Plan to deploy.Deployment.
There are two benefits to this change:
1. The name "Deployment" more accurately reflects the behavior of the
type, which is responsible for previewing or executing a deployment.
2. Renaming this type frees up the name "Plan" for use when addressing
#2318.
Just what it says on the tin. This is implemented by changing the
`GetPackageConfig` method of `ConfigSource` to return a `PropertyMap`
and ensuring that any secret config is represented by a `Secret`.
