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

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

The problem requiring a revert was creation (somehow) of /-containing
paths in compiler messages on Windows.  The new code deals with this
existing-but-unexpected behavior.

original was CL 465805
revert was CL 473795

this is the original, plus a correction for unexpected paths on Windows.

Change-Id: I786e875e704c2d7018c8248960f2ff7188cac3ef
Reviewed-on: https://go-review.googlesource.com/c/go/+/474015
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
Run-TryBot: David Chase <drchase@google.com>