Keep `Deps.cache` working even if used from a inside computation which is subsequently invalidated.

And failing test case.

@cmather -- I'm not sure if you intend it to be used like this, but I got super confused when it stopped triggering invalidations when the outer comp was re-run. Here's where I was using it: https://github.com/EventedMind/blaze-layout/blob/pass-regions-as-arguments/layout.js#L113-L120