Change mod ID regex to allow '.' in mod IDs

loader/src/main/java/net/neoforged/fml/loading/moddiscovery/ModInfo.java

@@ -29,7 +29,11 @@
 public class ModInfo implements IModInfo, IConfigurable {
     private static final Logger LOGGER = LogUtils.getLogger();
     private static final DefaultArtifactVersion DEFAULT_VERSION = new DefaultArtifactVersion("1");
-    private static final Pattern VALID_MODID = Pattern.compile("^[a-z][a-z0-9_]{1,63}$");
+    // Mod IDs must:
+    // - be from 2 to 64 characters long
+    // - contain only lowercase letters, digits, underscores, and periods, to satisfy both java module and resource location namespace requirements
+    // - each dot-separated section must start with a lowercase letter (they must be java identifiers)
+    private static final Pattern VALID_MODID = Pattern.compile("^(?=.{2,64}$)[a-z][a-z0-9_]*(\\.[a-z][a-z0-9_]*)*$");
     private static final Pattern VALID_NAMESPACE = Pattern.compile("^[a-z][a-z0-9_.-]{1,63}$");
     private static final Pattern VALID_VERSION = Pattern.compile("^\\d+.*");