go/types: use file start position as key for Info.FileVersions

go/types: use file start position as key for Info.FileVersions

Per discussion on CL 515135.

While at it, also use the file start position as key for the
internal map Checker.posVers.

Change-Id: I14e9b1ff9e8ee5e3ba5de181fc9c7ffc39f28261
Reviewed-on: https://go-review.googlesource.com/c/go/+/515656
Auto-Submit: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Griesemer <gri@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>