Split a lot of logic for Arch and Quilt mod metadata into new classes

src/main/java/dev/architectury/loom/metadata/ModMetadataFile.java

@@ -0,0 +1,12 @@
+package dev.architectury.loom.metadata;
+
+import java.util.List;
+
+import org.jetbrains.annotations.Nullable;
+
+import net.fabricmc.loom.configuration.ifaceinject.InterfaceInjectionProcessor;
+
+public interface ModMetadataFile {
+	@Nullable String getAccessWidener();
+	List<InterfaceInjectionProcessor.InjectedInterface> getInjectedInterfaces(@Nullable String modId);
+}