runtime: randomize package initialization order in race mode
This is one small step to force people to not depend on the order of
initialization of packages which are not explicitly ordered by import
directives. Similar to randomizing map iteration order, this makes
sure people aren't depending on the behavior of the current release,
so that we can change the order in future releases without breaking
everyone.
Maybe one day we can randomize always, but for now we do it just in
race mode. (We would need to measure the impact on startup time before
we enabled it always.)