Support incremental builds

[MichaelSimons]

With the introduction of the VMR, incremental build support is required to provide a productive development environment where developers want to work. Developers should be able to make repo source changes and rebuild quickly.

Today if you build the VMR, make a source change, and then rebuild, the changes code will not get detected and rebuilt. This is because of the current semaphore implementation. It requires intimate knowledge of the semaphore files and manual deletion in order to get completed build stages to rerun.

The ideal solution is a developer can build the vmr, make a src change anywere, rebuild and only the affected src is rebuilt along with all downstream dependencies.

T-Shirt Size: Medium/Large

[MichaelSimons]

This has some overlap with #928 but I think the VMR changes the UX requirements a bit in addition to the source-build state being quite different. #928 is in a way focused on a particular solution versus a desired UX.