- canonicalize package descriptors
- remove duplicate storage of file paths
- reduces (current) written index file by approx 3.5MB
(from 28434237B to 24686643B, or 13%)

- next step: untangle DAG (when serializing, using
gob, the index dag explodes into an index tree)

R=dsymonds
CC=golang-dev
https://golang.org/cl/4983042