Change mod ID regex to allow '.' in mod IDs (#203)

Also add mod ID format to the contract of getModId()

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

@@ -29,7 +29,7 @@
 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}$");
+    private static final Pattern VALID_MODID = Pattern.compile("^(?=.{2,64}$)[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+.*");
 

loader/src/main/java/net/neoforged/neoforgespi/language/IModInfo.java

@@ -27,6 +27,15 @@ public interface IModInfo {
      */
     IModLanguageLoader getLoader();
 
+    /**
+     * {@return the mod id}
+     * Mod IDs must satisfy the following requirements:
+     * <ul>
+     * <li>be from 2 to 64 characters long
+     * <li>contain only lowercase letters, digits, underscores, and periods
+     * <li>each dot-separated section must start with a lowercase letter
+     * </ul>
+     */
     String getModId();
 
     String getDisplayName();