The compiler inserts code that imports the exports of used packages.
It's important that we don't do this for the exports of used
debugOnly and prodOnly packages, because the inserted code will
fail at runtime if the packages are not linked in because we aren't
in dev mode or aren't in prod mode.
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.
The exact borderline between "Isobuild" and "the tool" is a little hazy,
but the idea is that "Isobuild" is the part that you could imagine being
embedded in a non-command-line build tool. It's not about commands or
UI, but just about building packages and apps.
A next step could be moving things that are strictly "the command-line
tool" into their own subdir, and leaving the top directory mostly just
for shared utilities like buildmessage. (Which could even go in a utils
subdir?)
