Split install plan into builder and struct (#955)
The `InstallPlan` does a lot of work in the constructor, which I tend to
feel is an anti-pattern. With cache refresh, it's also going to need to
be made `async`, so it really feels like it should be a clearer method
rather than an async, fallible constructor that does a bunch of IO. This
PR splits into a `Planner` (with a `build` method) and a `Plan`.
