Fix LoaderUtil saying there are duplicate classpath entries if there …

…are none. (#979)

Also futureproofs the checks just in case the class names are changed.

src/main/java/net/fabricmc/loader/impl/util/LoaderUtil.java

@@ -30,6 +30,8 @@
 
 public final class LoaderUtil {
 	private static final ConcurrentMap<Path, Path> pathNormalizationCache = new ConcurrentHashMap<>();
+	private static final String FABRIC_LOADER_CLASS = "net/fabricmc/loader/api/FabricLoader.class";
+	private static final String ASM_CLASS = "org/objectweb/asm/ClassReader.class";
 
 	public static String getClassFileName(String className) {
 		return className.replace('.', '/').concat(".class");
@@ -66,17 +68,21 @@ public static void verifyNotInTargetCl(Class<?> cls) {
 
 	public static void verifyClasspath() {
 		try {
-			List<URL> resources = Collections.list(LoaderUtil.class.getClassLoader().getResources("net/fabricmc/loader/api/FabricLoader.class"));
+			List<URL> resources = Collections.list(LoaderUtil.class.getClassLoader().getResources(FABRIC_LOADER_CLASS));
 
-			if (resources.size() != 1) {
+			if (resources.size() > 1) {
 				// This usually happens when fabric loader has been added to the classpath more than once.
 				throw new IllegalStateException("duplicate fabric loader classes found on classpath: " + resources.stream().map(URL::toString).collect(Collectors.joining(", ")));
+			} else if (resources.size() < 1) {
+				throw new AssertionError(FABRIC_LOADER_CLASS + " not detected on the classpath?! (perhaps it was renamed?)");
 			}
 
-			resources = Collections.list(LoaderUtil.class.getClassLoader().getResources("org/objectweb/asm/ClassReader.class"));
+			resources = Collections.list(LoaderUtil.class.getClassLoader().getResources(ASM_CLASS));
 
-			if (resources.size() != 1) {
+			if (resources.size() > 1) {
 				throw new IllegalStateException("duplicate ASM classes found on classpath: " + resources.stream().map(URL::toString).collect(Collectors.joining(", ")));
+			} else if (resources.size() < 1) {
+				throw new IllegalStateException("ASM not detected on the classpath (or perhaps " + ASM_CLASS + " was renamed?)");
 			}
 		} catch (IOException e) {
 			throw new UncheckedIOException("Failed to get resources", e);