use errors.Is for listPackage errors

main.go

@@ -1006,12 +1006,11 @@ func (tf *transformer) transformLinkname(localName, newName string) (string, str
 
 	lpkg, err := listPackage(pkgPath)
 	if err != nil {
-		// TODO(mvdan): use errors.As or errors.Is instead
+		if errors.Is(err, ErrNotFound) {
-		if strings.Contains(err.Error(), "path not found") {
 			// Probably a made up name like above, but with a dot.
 			return localName, newName
 		}
-		if strings.Contains(err.Error(), "refusing to list") {
+		if errors.Is(err, ErrNotDependency) {
 			fmt.Fprintf(os.Stderr,
 				"//go:linkname refers to %s - add `import _ %q` so garble can find the package",
 				newName, pkgPath)

shared.go

@@ -7,6 +7,7 @@ import (
 	"bytes"
 	"encoding/gob"
 	"encoding/json"
+	"errors"
 	"fmt"
 	"log"
 	"os"
@@ -314,6 +315,10 @@ var cannotObfuscate = map[string]bool{
 
 var listedRuntimeLinknamed = false
 
+var ErrNotFound = errors.New("not found")
+
+var ErrNotDependency = errors.New("not a dependency")
+
 // listPackage gets the listedPackage information for a certain package
 func listPackage(path string) (*listedPackage, error) {
 	if path == curPkg.ImportPath {
@@ -396,7 +401,7 @@ func listPackage(path string) (*listedPackage, error) {
 		return pkg, nil
 	}
 	if !ok {
-		return nil, fmt.Errorf("path not found in listed packages: %s", path)
+		return nil, fmt.Errorf("list %s: %w", path, ErrNotFound)
 	}
 
 	// Packages other than runtime can list any package,
@@ -420,5 +425,5 @@ func listPackage(path string) (*listedPackage, error) {
 		}
 	}
 
-	return nil, fmt.Errorf("refusing to list non-dependency package: %s", path)
+	return nil, fmt.Errorf("list %s: %w", path, ErrNotDependency)
 }