Refactor source layout

Introduce compat source sets and move all kotlin sources to the main directory

[no changelog]

src/main/kotlin/events/ReloadRegistrationEvent.kt

@@ -0,0 +1,7 @@
+package moe.nea.firmament.events
+
+import io.github.moulberry.repo.NEURepository
+
+data class ReloadRegistrationEvent(val repo: NEURepository) : FirmamentEvent() {
+    companion object : FirmamentEventBus<ReloadRegistrationEvent>()
+}