These are rarely used and can be computed on demand,
to make clear that they are never out of sync with the
lists in the non-internal Package fields.

Change-Id: I8c621dceaff1aeb39a3ed83f18e848adf14d7106
Reviewed-on: https://go-review.googlesource.com/56284Reviewed-by: David Crawshaw <crawshaw@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>