godoc: use file instead of file set for computing line info
godoc: use file instead of file set for computing line info

Reduces indexing time by ~10%; from ~44s to ~39s, best of 3 runs.

R=rsc
CC=golang-dev
http://codereview.appspot.com/3510041

Committer: Robert Griesemer <a6752097faebf1f02d80107d77a9be009089afe3@golang.org>
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go