cni-plugin-flannel: move to flannel directory

pkgs/applications/networking/cluster/cni/plugin-flannel.nix pkgs/tools/networking/flannel/plugin.nix

···
-{ lib, pkgs, buildGoModule, fetchFromGitHub }:
+{ lib, buildGoModule, fetchFromGitHub }:
 
 buildGoModule {
   pname = "cni-plugin-flannel";

pkgs/top-level/all-packages.nix

···
   fgallery = callPackage ../tools/graphics/fgallery { };
 
   flannel = callPackage ../tools/networking/flannel { };
+  cni-plugin-flannel = callPackage ../tools/networking/flannel/plugin.nix {};
 
   flare = callPackage ../games/flare {
     inherit (darwin.apple_sdk.frameworks) Cocoa;
···
 
   cni = callPackage ../applications/networking/cluster/cni {};
   cni-plugins = callPackage ../applications/networking/cluster/cni/plugins.nix {};
-  cni-plugin-flannel = callPackage ../applications/networking/cluster/cni/plugin-flannel.nix {};
 
   dnsname-cni = callPackage ../applications/networking/cluster/dnsname-cni {};