Subdirectories in different parts of the Zircon tree reusing the same
name is not a problem, and neither is //zircon/public/*/NAME reuse.
Fix the metadata-based target generation magic to distinguish the
category (i.e. subdirectory under //zircon/public/) so that names
can be reused across categories.

Bug: BLD-472 #done
Test: local build with //zircon/public/lib/blobfs export enabled
Change-Id: I592e52f524471a006bec2ff5942215ec277705dd