Until now, each scan of a file added a new file to the file set.
With this change, a file can be re-scanned using the same *token.File
w/o changing the file set. Eventually this will enable the re-use of
cached source code in godoc (for the fulltext index). At the moment,
source files are read over and over again from disk.

This is the first step in that direction.

R=r, rsc, r2
CC=golang-dev
https://golang.org/cl/4001041