copybara: use new Starlark Program API
copybara: use new Starlark Program API

Also, clarify flow: parse, compile, execute.

PiperOrigin-RevId: 328596504
Change-Id: Ia5fe32e5d914c4fe64efb7418a4a9897f6535e73

google/copybara,google/copybara,google/copybara