build_packages should be mostly standalone.  it has a _lot_ of option parsing, but is largely "just" a runner script for calling other programs.

here's the steps:
(1) implement chromite/scripts/build_packages.py that provides same functionality as src/scripts/build_packages
(2) update any callers to use new chromite path (use cs/ to find callers)
(3) delete build_packages from src/scripts/