[build] Allow targets in subdirectories of //bundles/assembly to reference sbase packages This is in preparation to migrating AIBs to Bazel. Bug: 505830390 Change-Id: Iaaaf5462724bdae5b27d9bad4111ff66a1faeb4a Reviewed-on: https://fuchsia-review.googlesource.com/c/third_party/sbase/+/1586548 Fuchsia-Auto-Submit: Aaron Wood <aaronwood@google.com> Reviewed-by: David Fang <fangism@google.com> Commit-Queue: Aaron Wood <aaronwood@google.com>