Make dependencies of target-files.zip conditional

Depend on the files going into the images instead of the images
themselves when the images are not prebuilts.  This will allow
paralellizing image generation with target-files.zip.

Test: treehugger
Test: m installclean && m target-files-package has same files before and after
Change-Id: If51d0f53602c08bd946ec34a2edae6f9d95f7bf8