buildFlatRemapper
private fun buildFlatRemapper(mappingsPath: String, classpath: List<File>, fromNs: String, toNs: String): FlatRemapper
Consume the SAME IMappingProvider tiny-remapper would (identical namespaces/descriptors), dumping its class/method/field mappings into flat tables keyed by "owner name desc". The MC hierarchy is read lazily from classpath (mc-symbols) inside FlatRemapper; the tables are immutable afterward.