Issue #2504513 by quietone, phenaproxima, mikeryan: Migrate URL aliases

core/modules/migrate_drupal/src/Tests/Table/d6/System.php

core/modules/path/migration_templates/d6_url_alias.yml

core/modules/path/migration_templates/d7_url_alias.yml

core/modules/path/src/Plugin/migrate/source/UrlAliasBase.php

core/modules/path/src/Plugin/migrate/source/d6/UrlAlias.php

core/modules/path/src/Plugin/migrate/source/d7/UrlAlias.php

...modules/path/src/Tests/Migrate/d7/MigrateUrlAliasTest.php

.../modules/path/tests/src/Unit/Migrate/UrlAliasTestBase.php

core/modules/path/tests/src/Unit/Migrate/d6/UrlAliasTest.php

core/modules/path/tests/src/Unit/Migrate/d7/UrlAliasTest.php