Check each dependency's validity again right before installing it,
because a GC during preceding installations can theoretically trigger
invalidation for some dependency kinds.

Also inline the IsSane checkers into the constructors.

R=jarin@chromium.org

Change-Id: I1331dee27f01e8fd07cb953dddfed72fd1841559
Reviewed-on: https://chromium-review.googlesource.com/1161933Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Commit-Queue: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54920}