Split build into likely changed and likely not changed

[davidmorgan]

The most common interactive change is that one file changes and the generated files that have it as a (transitive) primary input need rebuilding. This can be a much smaller set than transitive reverse imports.

In watch mode: it should be fairly straightforward to build these first and flush to disk, then continue to the rest of the build. If nothing else needs rebuilding, this should work much better for interactive use.

It would also be interesting to make the second part of the build abort if further changes are made, to restart the fast build sooner.