Porch: PackageVariant should set a readinessgate

[johnbelamaric]

Expected behavior

If PackageVariant fails to inject or otherwise properly mutate the Draft it creates, we need to prevent the package from being approved. This could be done by adding a False condition and readinessGate immediately after cloning (or better yet, implementing a porch server feature that allows that to be done atomically).

Actual behavior

Because we cannot atomically "clone and apply PV mutations", the actual package revision process happens in two independent stages: clone, then apply mutations. If applying the mutations fails, the newly cloned Draft is still there, and unless you check the PV conditions, it looks fine and can be merged (in fact the auto-approval controller we built for Nephio goes ahead and approves it, since it only inspect the package revision, and doesn't know where it came from).

[johnbelamaric]

As a workaround for this, the approval controller now checks the PV readiness status, and so do some of our other controllers. But that wouldn't help a human user.