go/packages/packagestest: fix MustCopyFileTree so that file fragments are always slash form

Change-Id: I866384c2b382cbf3e839f0ef826eea654319fa5d
Reviewed-on: https://go-review.googlesource.com/c/tools/+/176219
Run-TryBot: Ian Cottrell <iancottrell@google.com>
Reviewed-by: Rebecca Stambler <rstambler@golang.org>
diff --git a/go/packages/packagestest/export.go b/go/packages/packagestest/export.go
index cb9e6be..7cc3b54 100644
--- a/go/packages/packagestest/export.go
+++ b/go/packages/packagestest/export.go
@@ -263,7 +263,7 @@
 		if err != nil {
 			return err
 		}
-		result[fragment] = Copy(path)
+		result[filepath.ToSlash(fragment)] = Copy(path)
 		return nil
 	}); err != nil {
 		log.Panic(fmt.Sprintf("MustCopyFileTree failed: %v", err))