handle duplicate URIs and recipes with no URIs

There are some recipes (like libxml-parser-perl) where bad bbappends add duplicate SRC_URI entries. In such case UnpackTracer fails because it identifies UrlNode children by their URI and therefore it tries to restart nodes that have already been finished. We should use a unique index to identify UrlNodes instead, so that we can handle such corner cases

There are some recipes (like qemuwrapper-cross) that do not have any SRC_URI. In such case, UnpackTracer is not even called so we need to handle that in bbtracer.bbclass.