Move internal packages to an internal folder

For reference: https://go.dev/doc/go1.4#internalpackages

ui/form/auth.go

@@ -1,33 +0,0 @@
-// SPDX-FileCopyrightText: Copyright The Miniflux Authors. All rights reserved.
-// SPDX-License-Identifier: Apache-2.0
-
-package form // import "miniflux.app/v2/ui/form"
-
-import (
-	"net/http"
-
-	"miniflux.app/v2/errors"
-)
-
-// AuthForm represents the authentication form.
-type AuthForm struct {
-	Username string
-	Password string
-}
-
-// Validate makes sure the form values are valid.
-func (a AuthForm) Validate() error {
-	if a.Username == "" || a.Password == "" {
-		return errors.NewLocalizedError("error.fields_mandatory")
-	}
-
-	return nil
-}
-
-// NewAuthForm returns a new AuthForm.
-func NewAuthForm(r *http.Request) *AuthForm {
-	return &AuthForm{
-		Username: r.FormValue("username"),
-		Password: r.FormValue("password"),
-	}
-}