This was part of an attempt to add "git describe" to zig build
without re-building too much. This doesn't yet work as zig upstream
changes are are also needed, but I think this was a sensible refactor
even in isolation, so breaking it out.
"API dispatch" doesn't depend on any ui event stuff nor on
version, that was just an accident of the metadata collection
being crammed into the same file.
Remove "manual invocation" instruction. this is going to bitrot anyway.
Both cmake and build.zig allows you to extract the command line of a
step, so you can debug it separately.
