This allows users of go/packages to replace the contents of already
existing files, to support use-cases such as unsaved files in editors.
BREAKING CHANGE: This CL changes the signature of the function provided
to Config.ParseFile.
Change-Id: I6ce50336060832679e9f64f8d201b44651772e0b
Reviewed-on: https://go-review.googlesource.com/c/139798
Reviewed-by: Alan Donovan <adonovan@google.com>