Mirrors: skip attempts to fetch BundlePackages

BundlePackages use a noop fetch strategy. The mirror logic was assuming
that the fetcher had a resource to cach after performing a fetch. This adds
a special check to skip caching if the stage is associated with a
BundleFetchStrategy. Note that this should allow caching resources
associated with BundlePackages.

lib/spack/spack/stage.py

@@ -492,6 +492,16 @@ def cache_local(self):
 
     def cache_mirror(self, stats):
         """Perform a fetch if the resource is not already cached"""
+        if isinstance(self.default_fetcher, fs.BundleFetchStrategy):
+            # BundleFetchStrategy has no source to fetch. The associated
+            # fetcher does nothing but the associated stage may still exist.
+            # There is currently no method available on the fetcher to
+            # distinguish this ('cachable' refers to whether the fetcher
+            # refers to a resource with a fixed ID, which is not the same
+            # concept as whether there is anything to fetch at all) so we
+            # must examine the type of the fetcher.
+            return
+
         dst_root = spack.caches.mirror_cache.root
         absolute_storage_path = os.path.join(
             dst_root, self.mirror_paths.storage_path)