libgo: refactor code to enumerate stdlib packages

Move the list of libgo, gotool, and check-target packages into
separate files, then read the file contents as part of the build
process on the fly. This is intended to enable other build tooling to
share the canonical list of target packages (avoid duplication).

Change-Id: Iefc4a88843a74a4d7e465cf94e53755b0039e5c5
Reviewed-on: https://go-review.googlesource.com/89515
Reviewed-by: Ian Lance Taylor <iant@golang.org>
5 files changed