swift234

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

 - This adds a new argument `--only-execute <name>` which can be used by
   `build-script` to invoke the `build-script-impl` to perform each different
   action, while moving the high-level operation loop into the `build-script`
   itself. This should make incremental refactoring of the individual actions
   into `build-script` easier.

 - This is part of SR-237.