cmd/go/internal/work: remove '_test' from import paths in stacktraces when -trimpath is specified

ExampleFrames with -trimpath failed since the content of Frame's File
changed when -trimpath is specified.

This CL fixes the issue by adding a new field OrigImportPath to
PackageInternal, which represents the original import path before adding
'_test' suffix for an external test package, and always using it to
create paths for the build tools.

Fixes golang/go#43380

Change-Id: Ibbc947eb3ae08a7ba81f13f03af67c8745b5c69f
Reviewed-on: https://go-review.googlesource.com/c/go/+/279440


Run-TryBot: Hajime Hoshi <hajimehoshi@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Hajime Hoshi <hajimehoshi@gmail.com>
Reviewed-by: Jay Conrod <jayconrod@google.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>