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.