Issue #2178703 by ultimike, chx, penyaskito: Migrate D6 menu links.

core/lib/Drupal/Core/Path/PathValidator.php

core/lib/Drupal/Core/Path/PathValidatorInterface.php

core/modules/migrate/src/Plugin/migrate/process/Route.php

...drupal/config/install/migrate.migration.d6_menu_links.yml

...ate_drupal/config/schema/migrate_drupal.source.schema.yml

.../migrate_drupal/src/Plugin/migrate/source/d6/MenuLink.php

...modules/migrate_drupal/src/Tests/Dump/Drupal6MenuLink.php

...odules/migrate_drupal/src/Tests/d6/MigrateDrupal6Test.php

...dules/migrate_drupal/src/Tests/d6/MigrateMenuLinkTest.php

...te_drupal/tests/src/Unit/source/d6/MenuLinkSourceTest.php

core/tests/Drupal/Tests/Core/Path/PathValidatorTest.php