allow nested resources

This solution doesn't really make me happy, but does seem to work.  It
sorts the resources by the length of the string representing their
destination.  Since any nested resource must contain another resource's
name in its path, it seems that should work, but there should be a
better way to do this.

lib/spack/spack/package.py

@@ -697,7 +697,9 @@ def _expand_archive(stage, name=self.name):
         ##########
         # Stage resources in appropriate path
         resources = self._get_resources()
-        for resource in resources:
+        # TODO: this is to allow nested resources, a better solution would be
+        # good
+        for resource in sorted(resources, key=lambda res: len(res.destination)):
             stage = resource.fetcher.stage
             _expand_archive(stage, resource.name)
             # Turn placement into a dict with relative paths