Fix for files not being downloaded when the don't exist. Closes #122

@asiekierka im not sure if this is the correct fix so could you please verify, thanks.

src/main/java/net/fabricmc/loom/util/StaticPathWatcher.java

@@ -50,6 +50,9 @@ public final class StaticPathWatcher {
     }
 
     public boolean hasFileChanged(Path filePath) {
+    	if(!Files.exists(filePath)){
+    		return true;
+	    }
         WatchKey key;
         while ((key = service.poll()) != null) {
             for (WatchEvent<?> event : key.pollEvents()) {