Refactor source layout

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

[no changelog]

src/main/kotlin/features/texturepack/BakedModelExtra.kt

@@ -0,0 +1,9 @@
+
+package moe.nea.firmament.features.texturepack
+
+import net.minecraft.client.render.model.BakedModel
+
+interface BakedModelExtra {
+    fun getHeadModel_firmament(): BakedModel?
+    fun setHeadModel_firmament(headModel: BakedModel?)
+}