Allow add to merge into existing package

main.go

@@ -247,8 +247,9 @@ func addCmd(args []string) error {
 	}
 
 	pkgDir := filepath.Join(repo, pkgName)
+	pkgExists := false
 	if _, err := os.Stat(pkgDir); err == nil {
-		return fmt.Errorf("package %q already exists", pkgName)
+		pkgExists = true
 	} else if !errors.Is(err, os.ErrNotExist) {
 		return err
 	}
@@ -259,11 +260,15 @@ func addCmd(args []string) error {
 	}
 
 	configPath := filepath.Join(pkgDir, configFileName)
-	if err := writeConfig(configPath, targetRoot); err != nil {
-		return err
+	if !pkgExists {
+		if err := writeConfig(configPath, targetRoot); err != nil {
+			return err
+		}
 	}
-
 	if info.IsDir() {
+		if pkgExists {
+			return errors.New("cannot merge a directory into an existing package yet")
+		}
 		if err := moveDirContents(absPath, filesDir); err != nil {
 			return err
 		}
@@ -280,6 +285,11 @@ func addCmd(args []string) error {
 		if err := os.MkdirAll(filepath.Dir(destPath), 0o755); err != nil {
 			return err
 		}
+		if _, err := os.Stat(destPath); err == nil {
+			return fmt.Errorf("destination already exists: %s", destPath)
+		} else if !errors.Is(err, os.ErrNotExist) {
+			return err
+		}
 		if err := os.Rename(absPath, destPath); err != nil {
 			return err
 		}