It's very useful to be able to include code only in production. It's
useful for React integration, but presumably also for many apps.
* Add prodOnly boolean flag alongside debugOnly in Package.describe
* Packages the set prodOnly to true auto-depend on isobuild:prod-only
(making them error in the old tool where prodOnly isn't supported)
* The `includeDebug` boolean build option is replaced by a string
named buildMode, which can be 'development' or 'production', just
like minifyMode.
Tested by self-test.
