We split aquiring a "handle" from reading a files contents so that we can do the
former eagerly and the latter lazily.
We also "version" the handles so that the same file at different versions is a
different handle.
Change-Id: I06cc346d4b4c77d784aa454702c54689f2f177e0
Reviewed-on: https://go-review.googlesource.com/c/tools/+/179917
Reviewed-by: Rebecca Stambler <rstambler@golang.org>