Fetching from URLs falls back to mirrors if they exist (#13881)

Users can now list mirrors of the main url in packages.

- [x] Instead of just a single `url` attribute, users can provide a list (`urls`) in the package, and these will be tried by in order by the fetch strategy.

- [x] To handle one of the most common mirror cases, define a `GNUMirrorPackage` mixin to handle all the standard GNU mirrors.  GNU packages can set `gnu_mirror_path` to define the path within a mirror, and the mixin handles setting up all the requisite GNU mirror URLs.

- [x] update all GNU packages in `builtin` to use the `GNUMirrorPackage` mixin.

var/spack/repos/builtin/packages/cvs/package.py

@@ -7,10 +7,10 @@
 from spack import *
 
 
-class Cvs(AutotoolsPackage):
+class Cvs(AutotoolsPackage, GNUMirrorPackage):
     """CVS a very traditional source control system"""
     homepage = "http://www.nongnu.org/cvs/"
-    url      = "https://ftpmirror.gnu.org/non-gnu/cvs/source/feature/1.12.13/cvs-1.12.13.tar.bz2"
+    gnu_mirror_path = "non-gnu/cvs/source/feature/1.12.13/cvs-1.12.13.tar.bz2"
 
     version('1.12.13', sha256='78853613b9a6873a30e1cc2417f738c330e75f887afdaf7b3d0800cb19ca515e')