Do not try to translate classes in META-INF/versions

In a multirelease-jar environment, there could be classes under
META-INF/versions, which retrolambda would try to translate.

As per definition, these are Java classes newer than Java 1.8, so they
shouldn't be processed at all, especially since it is expected that
there is at least some version (hopefully 1.8 or older) of that class in
the path that is not prefixed by META-INF/versions.

Extend FilteringFileVisitor to never visit /META-INF/versions, and
modify FilteringFileVisitor so it's usable in all cases, even when no
other filtering is required.

Author: kohlschuetter

retrolambda/src/main/java/net/orfjackal/retrolambda/files/FilteringFileVisitor.java

@@ -15,7 +15,7 @@ public class FilteringFileVisitor implements FileVisitor<Path> {
     private final FileVisitor<? super Path> target;
 
     public FilteringFileVisitor(Collection<Path> fileFilter, FileVisitor<Path> target) {
-        this.fileFilter = new HashSet<>(fileFilter);
+        this.fileFilter = fileFilter == null ? null : new HashSet<>(fileFilter);
         this.target = target;
     }
 
@@ -26,12 +26,15 @@ public FileVisitResult postVisitDirectory(Path dir, IOException exc) throws IOEx
 
     @Override
     public FileVisitResult preVisitDirectory(Path dir, BasicFileAttributes attrs) throws IOException {
+        if (dir.toString().endsWith("/META-INF/versions")) {
+            return FileVisitResult.SKIP_SUBTREE;
+        }
         return target.preVisitDirectory(dir, attrs);
     }
 
     @Override
     public FileVisitResult visitFile(Path file, BasicFileAttributes attrs) throws IOException {
-        if (fileFilter.contains(file)) {
+        if (fileFilter == null || fileFilter.contains(file)) {
             return target.visitFile(file, attrs);
         } else {
             return FileVisitResult.CONTINUE;