Issue #2534156 by mikeryan: Move module-specific migration support into the path module

core/modules/path/migration_templates/d6_url_alias.yml

.../modules/path/src/Plugin/migrate/destination/UrlAlias.php

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

...modules/path/src/Tests/Migrate/d6/MigrateUrlAliasTest.php

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