fix path resolution for mirror packages, especially with dependency fetching

lib/spack/spack/cmd/mirror.py

@@ -136,6 +136,7 @@ def mirror_create(args):
     if args.dependencies:
         new_specs = set()
         for spec in specs:
+            spec.concretize()
             for s in spec.traverse():
                 new_specs.add(s)
         specs = list(new_specs)

lib/spack/spack/mirror.py

@@ -146,7 +146,7 @@ def create(path, specs, **kwargs):
         stage = None
         try:
             # create a subdirectory for the current package@version
-            archive_path = os.path.abspath(join_path(path, mirror_archive_path(spec)))
+            archive_path = os.path.abspath(join_path(mirror_root, mirror_archive_path(spec)))
             subdir = os.path.dirname(archive_path)
             mkdirp(subdir)