Move internal packages to an internal folder

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

ui/form/category.go

@@ -1,22 +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"
-)
-
-// CategoryForm represents a feed form in the UI
-type CategoryForm struct {
-	Title        string
-	HideGlobally string
-}
-
-// NewCategoryForm returns a new CategoryForm.
-func NewCategoryForm(r *http.Request) *CategoryForm {
-	return &CategoryForm{
-		Title:        r.FormValue("title"),
-		HideGlobally: r.FormValue("hide_globally"),
-	}
-}