Move internal packages to an internal folder

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

internal/validator/validator.go

@@ -0,0 +1,66 @@
+// SPDX-FileCopyrightText: Copyright The Miniflux Authors. All rights reserved.
+// SPDX-License-Identifier: Apache-2.0
+
+package validator // import "miniflux.app/v2/internal/validator"
+
+import (
+	"errors"
+	"fmt"
+	"net/url"
+	"regexp"
+
+	"miniflux.app/v2/internal/locale"
+)
+
+// ValidationError represents a validation error.
+type ValidationError struct {
+	TranslationKey string
+}
+
+// NewValidationError initializes a validation error.
+func NewValidationError(translationKey string) *ValidationError {
+	return &ValidationError{TranslationKey: translationKey}
+}
+
+func (v *ValidationError) String() string {
+	return locale.NewPrinter("en_US").Printf(v.TranslationKey)
+}
+
+func (v *ValidationError) Error() error {
+	return errors.New(v.String())
+}
+
+// ValidateRange makes sure the offset/limit values are valid.
+func ValidateRange(offset, limit int) error {
+	if offset < 0 {
+		return fmt.Errorf(`Offset value should be >= 0`)
+	}
+
+	if limit < 0 {
+		return fmt.Errorf(`Limit value should be >= 0`)
+	}
+
+	return nil
+}
+
+// ValidateDirection makes sure the sorting direction is valid.
+func ValidateDirection(direction string) error {
+	switch direction {
+	case "asc", "desc":
+		return nil
+	}
+
+	return fmt.Errorf(`Invalid direction, valid direction values are: "asc" or "desc"`)
+}
+
+// IsValidRegex verifies if the regex can be compiled.
+func IsValidRegex(expr string) bool {
+	_, err := regexp.Compile(expr)
+	return err == nil
+}
+
+// IsValidURL verifies if the provided value is a valid absolute URL.
+func IsValidURL(absoluteURL string) bool {
+	_, err := url.ParseRequestURI(absoluteURL)
+	return err == nil
+}