Separate Go discovery configuration from diagnostics

internal/gotool/gotool.go

@@ -187,18 +187,18 @@ func discoveryEnvironment() map[string]string {
 }
 
 func discover(ctx context.Context, directory string, environment map[string]string, args ...string) ([]byte, error) {
-	captured := &combinedWriter{}
+	var configuration, diagnostics bytes.Buffer
 	err := process.Run(ctx, process.Invocation{
 		Args:   args,
 		Dir:    directory,
 		Env:    environment,
-		Stdout: captured,
-		Stderr: captured,
+		Stdout: &configuration,
+		Stderr: &diagnostics,
 	})
-	contents := captured.Bytes()
+	contents := configuration.Bytes()
 	if err != nil {
-		if len(contents) != 0 {
-			return contents, fmt.Errorf("%s: %w: %s", strings.Join(args, " "), err, strings.TrimSpace(string(contents)))
+		if output := strings.TrimSpace(configuration.String() + diagnostics.String()); output != "" {
+			return contents, fmt.Errorf("%s: %w: %s", strings.Join(args, " "), err, output)
 		}
 		return contents, fmt.Errorf("%s: %w", strings.Join(args, " "), err)
 	}