Move internal packages to an internal folder

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

internal/ui/form/api_key.go

@@ -0,0 +1,31 @@
+// SPDX-FileCopyrightText: Copyright The Miniflux Authors. All rights reserved.
+// SPDX-License-Identifier: Apache-2.0
+
+package form // import "miniflux.app/v2/internal/ui/form"
+
+import (
+	"net/http"
+
+	"miniflux.app/v2/internal/errors"
+)
+
+// APIKeyForm represents the API Key form.
+type APIKeyForm struct {
+	Description string
+}
+
+// Validate makes sure the form values are valid.
+func (a APIKeyForm) Validate() error {
+	if a.Description == "" {
+		return errors.NewLocalizedError("error.fields_mandatory")
+	}
+
+	return nil
+}
+
+// NewAPIKeyForm returns a new APIKeyForm.
+func NewAPIKeyForm(r *http.Request) *APIKeyForm {
+	return &APIKeyForm{
+		Description: r.FormValue("description"),
+	}
+}