go/importer: report import path if package is not found

Fixes #20230.

Change-Id: I2e9b9e9d2540eb66c8411ac7910962933bc2c0e9
Reviewed-on: https://go-review.googlesource.com/42870
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>

src/go/internal/gcimporter/gcimporter.go

@@ -43,6 +43,7 @@ func FindPkg(path, srcDir string) (filename, id string) {
 		}
 		bp, _ := build.Import(path, srcDir, build.FindOnly|build.AllowBinary)
 		if bp.PkgObj == "" {
+			id = path // make sure we have an id to print in error message
 			return
 		}
 		noext = strings.TrimSuffix(bp.PkgObj, ".a")
@@ -89,7 +90,7 @@ func Import(packages map[string]*types.Package, path, srcDir string) (pkg *types
 		if path == "unsafe" {
 			return types.Unsafe, nil
 		}
-		err = fmt.Errorf("can't find import: %s", id)
+		err = fmt.Errorf("can't find import: %q", id)
 		return
 	}