Fix usage of wrong lock object (#163)

This changes the lock object to `lock`, which every other method is locking on.

pkl-core/src/main/java/org/pkl/core/packages/PackageResolvers.java

@@ -554,7 +554,7 @@ class PackageResolvers {
     private FileSystem getZipFileSystem(PackageAssetUri uri, @Nullable Checksums checksums)
         throws IOException, SecurityManagerException {
       var packageUri = uri.getPackageUri();
-      synchronized (fileSystems) {
+      synchronized (lock) {
         var fs = fileSystems.get(packageUri);
         if (fs == null) {
           var metadata = getDependencyMetadata(packageUri, checksums);