Have fetch use list_url

This PR allows archive file retrieval from urls derived from the
`list_url` setting in a package file. This allows for continued
retrieval of checksummed archive files even when they are moved to a new
remote location when a package is updated upstream.

lib/spack/spack/stage.py

@@ -37,6 +37,7 @@
 import spack.config
 import spack.fetch_strategy as fs
 import spack.error
+from spack.version import Version
 
 STAGE_PREFIX = 'spack-stage-'
 
@@ -306,6 +307,18 @@ def fetch(self, mirror_only=False):
                 fetchers.insert(0, fs.URLFetchStrategy(url, digest))
             fetchers.insert(0, spack.cache.fetcher(self.mirror_path, digest))
 
+            # Look for the archive in list_url
+            archive_version = spack.url.parse_version(self.default_fetcher.url)
+            package_name = os.path.dirname(self.mirror_path)
+            pkg = spack.repo.get(package_name)
+            versions = pkg.fetch_remote_versions()
+            try:
+                url_from_list = versions[Version(archive_version)]
+                fetchers.append(fs.URLFetchStrategy(url_from_list, digest))
+            except KeyError:
+                tty.msg("Can not find version %s in url_list" %
+                        archive_version)
+
         for fetcher in fetchers:
             try:
                 fetcher.set_stage(self)