Handle the case of a Multi-Release Jar not actually containing Multi-Release Classes (#42)

Seen a couple of people encounter this in mod dev (when adding
dependencies):

SJH considers a Jar to be `Multi-Release` based on the `Manifest`

<https://github.com/McModLauncher/securejarhandler/blob/96a7b6a8fe1f1117e01dfd94c640757154372a68/src/main/java/cpw/mods/jarhandling/impl/Jar.java#L124>

If it is then it resolves a path to `/META-INF/versions`

<https://github.com/McModLauncher/securejarhandler/blob/96a7b6a8fe1f1117e01dfd94c640757154372a68/src/main/java/cpw/mods/jarhandling/impl/Jar.java#L126>

And tries to walk it

<https://github.com/McModLauncher/securejarhandler/blob/96a7b6a8fe1f1117e01dfd94c640757154372a68/src/main/java/cpw/mods/jarhandling/impl/Jar.java#L127>

This works for most Jars, however it does not consider the case where a
Jar is erroneously marked as `Multi-Release`, with `/META-INF/versions`
**not** existing.

If this is the case, `Files.walk` will throw a
`UnionFileSystem$NoSuchFileException` which is then wrapped and rethrown
as a `UncheckedIOException`

<https://github.com/McModLauncher/securejarhandler/blob/96a7b6a8fe1f1117e01dfd94c640757154372a68/src/main/java/cpw/mods/jarhandling/impl/Jar.java#L138>

Attached Example Stacktrace,
<https://gist.github.com/AterAnimAvis/35ded98d52a78c2f257d59173c86366c>

---

Current implementation of a fix is from a bit of spitballing [in
Discord](https://discord.com/channels/313125603924639766/922237746460893234/1129415771567689738)

Author: AterAnimAvis

src/main/java/cpw/mods/jarhandling/impl/JarContentsImpl.java

@@ -115,6 +115,8 @@ private Map<Path, Integer> readMultiReleaseInfo() {
         }
 
         var vers = filesystem.getRoot().resolve("META-INF/versions");
+        if (!Files.isDirectory(vers)) return Map.of();
+
         try (var walk = Files.walk(vers)) {
             Map<Path, Integer> pathToJavaVersion = new HashMap<>();
             walk

src/test/java/cpw/mods/jarhandling/impl/TestMultiRelease.java

@@ -28,6 +28,13 @@ public void testMultiRelease() {
         Assertions.assertNotEquals("too new", bContents.strip());
     }
 
+    @Test
+    public void testMultiReleaseNoVersions() {
+        Path rootDir = Paths.get("src", "test", "resources", "multirelease-noversions");
+        // Jars marked with Multi-Release but don't actually have a versions folder should not throw
+        Assertions.assertDoesNotThrow(() -> SecureJar.from(rootDir));
+    }
+
     private static String readString(SecureJar jar, String file) {
         // Note: we must read the jar through the module data provider for version-specific files to be used
         try (var is = jar.moduleDataProvider().open(file).get()) {

src/test/resources/multirelease-noversions/META-INF/MANIFEST.MF

@@ -0,0 +1 @@
+Multi-Release: true