Consider package-level conditions in gnrt

This ensures that we don't try to build a package if its dependencies
aren't available. Combined with previous changes to Condition.rs, this
also means we're doing less unnecessary work building packages that
aren't needed for a given condition.

Note: written by lukasza, uploaded by dloehr.

Bug: 402096443, 417481504
Change-Id: I0e53494b1e2377cb90ed45b24c0a0173cae0a4b9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6543427
Reviewed-by: Łukasz Anforowicz <lukasza@chromium.org>
Commit-Queue: Devon Loehr <dloehr@google.com>
Auto-Submit: Devon Loehr <dloehr@google.com>
Commit-Queue: Łukasz Anforowicz <lukasza@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1460157}