This lets godoc implementations provide a more efficient means
of getting this information, without reading files and parsing the
package docs.
This is especially important when the files themselves don't
actually exist and the VFS is synthesizing them on demand
(e.g. protocol buffer files -> their generated *.pb.go files).
This means corpus.Init can run quickly, without generating
every protocol file in a large corpus (or fetching it from a
cache).
In the future, this hook could also be used for caching the summaries of
regular packages.
R=golang-dev, adg
CC=golang-dev
https://golang.org/cl/19440043
Massive win for high-latency network filesystems.
Also benefits the local disk/ssd case too, though.
R=golang-dev, bgarcia
CC=golang-dev
https://golang.org/cl/18650043
