Rewrite bp2build-sync.sh in python. This CL transliterates bp2build-sync.sh to Python, paving the way for smarter extensions (e.g. json extension) and more logic for moving BUILD files in and out of the source tree. Test: bp2build-incremental Test: demo.sh full Change-Id: Ib217dce81fda1937270b8d7ab174598f4e069142