cmd/ent: Add check that model file doesn't already exist on ent init (#2307)

Previously if you run  it would overwrite an existing file  if that already existed. Now  checks if that file already exists, and returns an error if so.

cmd/internal/base/base.go

@@ -184,6 +184,9 @@ func initEnv(target string, names []string) error {
 		if err := gen.ValidSchemaName(name); err != nil {
 			return fmt.Errorf("init schema %s: %w", name, err)
 		}
+		if fileExists(target, name) {
+			return fmt.Errorf("init schema %s: already exists", name)
+		}
 		b := bytes.NewBuffer(nil)
 		if err := tmpl.Execute(b, name); err != nil {
 			return fmt.Errorf("executing template %s: %w", name, err)
@@ -213,6 +216,12 @@ func createDir(target string) error {
 	return nil
 }
 
+func fileExists(target, name string) bool {
+	var _, err = os.Stat(filepath.Join(target, strings.ToLower(name+".go")))
+
+	return err == nil
+}
+
 // schema template for the "init" command.
 var tmpl = template.Must(template.New("schema").
 	Parse(`package schema