mirror:// support

Currently mirror:// URLs are only supported in `nix-prefetch-url`. It might be useful to support them in other places, like `builtins.fetchURL` or tarball flake inputs. The main problem is where to get the mirror list. Currently `nix-prefetch-url` gets it from `<nixpkgs/pkgs/build-support/fetchurl/mirrors.nix>`, which may not exist or may be out of date. We could fetch it automatically like we do with the flake registry.