Only use list if list_url set

This commit will make urls from list_url only checked if `list_url` is
set in the package file. This makes more sense as there is no need to
check for those if the attribute is not present. If `url` is present and
`list_url` is not then it would result in the same url. If
`url_for_version` is used then that will not work anyway.

lib/spack/spack/stage.py

@@ -37,7 +37,7 @@
 import spack.config
 import spack.fetch_strategy as fs
 import spack.error
-from spack.version import Version
+from spack.version import *
 
 STAGE_PREFIX = 'spack-stage-'
 
@@ -308,6 +308,7 @@ def fetch(self, mirror_only=False):
             archive_version = spack.url.parse_version(self.default_fetcher.url)
             package_name = os.path.dirname(self.mirror_path)
             pkg = spack.repo.get(package_name)
+            if pkg.list_url is not None:
                 versions = pkg.fetch_remote_versions()
                 try:
                     url_from_list = versions[Version(archive_version)]