fix: Isolate mixins in alternative source sets

src/main/kotlin/util/compatloader/CompatMeta.kt

@@ -0,0 +1,48 @@
+package moe.nea.firmament.util.compatloader
+
+import java.util.ServiceLoader
+import moe.nea.firmament.events.subscription.SubscriptionList
+import moe.nea.firmament.init.AutoDiscoveryPlugin
+import moe.nea.firmament.util.ErrorUtil
+
+/**
+ * Declares the compat meta interface for the current source set.
+ * This is used by [CompatLoader], [SubscriptionList], and [AutoDiscoveryPlugin]. Annotate a [ICompatMeta] object with
+ * this.
+ */
+annotation class CompatMeta
+
+interface ICompatMetaGen {
+	fun owns(className: String): Boolean
+	val meta: ICompatMeta
+}
+
+interface ICompatMeta {
+	fun shouldLoad(): Boolean
+
+	companion object {
+		val allMetas = ServiceLoader
+			.load(ICompatMetaGen::class.java)
+			.toList()
+
+		fun shouldLoad(className: String): Boolean {
+			// TODO: replace this with a more performant package lookup
+			val meta = if (ErrorUtil.aggressiveErrors) {
+				val fittingMetas = allMetas.filter { it.owns(className) }
+				require(fittingMetas.size == 1) { "Orphaned or duplicate owned class $className (${fittingMetas.map { it.meta }}). Consider adding a @CompatMeta object." }
+				fittingMetas.single()
+			} else {
+				allMetas.firstOrNull { it.owns(className) }
+			}
+			return meta?.meta?.shouldLoad() ?: true
+		}
+	}
+}
+
+object CompatHelper {
+	fun isOwnedByPackage(className: String, vararg packages: String): Boolean {
+		// TODO: create package lookup structure once
+		val packageName = className.substringBeforeLast('.')
+		return packageName in packages
+	}
+}