cmd/go: extend path shortening to all paths in messages

cmd/go: extend path shortening to all paths in messages

The previous code only shortened the directory name for files
in the directory being compiled (and $WORK and runtime/std).

This extends the shortening to all file names.

Change-Id: I6abef6141d57036d893420b82e01ed0fbb637788
Reviewed-on: https://go-review.googlesource.com/c/go/+/465805
Reviewed-by: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: David Chase <drchase@google.com>