Amake: cached builds of top-level targets

With a typical build tool, if a top-level target must be updated, intermediate targets must be built from sources, and then combined to build the top-level target. The enhancements described here allow a top-level target to be fetched from a shared cache, without building, or even fetching its intermediate-target dependencies. Thus, a developer's workspace may need only contain sources and top-level targets. This reduces build time, reduces network traffic, and saves disk space.