swift234

[build-script] Add support for using isolated -impl actions.

 - This change moves the top-level invocation driver loop into `build-script`
   and uses the `-impl` script to perform each individual action. Once landed
   and enabled, this will enable us to migrate the individual pieces of the
   `-impl` script into Python code in an incremental fashion.

 - This also introduces stub product definitions for each of the different projects
   we manage.

 - This works, but is disabled by default (`--no-legacy-impl`) because it
   severely impacts the performance of null builds (4x slower, currently) due to
   the `build-script-impl` parsing overhead. If only we had a JITing bash
   implementation...