flake: remove nonstandard top-level flake attrs

Silences warnings in `nix flake check`.

- Define `mkNbPkgs` under attr `lib`.
  `lib` still triggers a warning, but it is expected to be a standard
  flake attr in a future Nix release.

- Define `nbPkgs` under attr `legacyPackages`.
  This also has the advantage to make its contents more easily
  accessible via the nix CLI. Example:
  nix eval nix-bitcoin#nbPkgs.modulesPkgs.clightning.version

flake.nix

@@ -15,22 +15,24 @@
       supportedSystems = [ "x86_64-linux" "i686-linux" "aarch64-linux" ];
     in
     rec {
+      lib = {
         mkNbPkgs = {
           system
           , pkgs ? import nixpkgs { inherit system; }
           , pkgsUnstable ? import nixpkgsUnstable { inherit system; }
         }:
           import ./pkgs { inherit pkgs pkgsUnstable; };
+      };
 
       overlay = final: prev: let
-        nbPkgs = mkNbPkgs { inherit (final) system; pkgs = final; };
+        nbPkgs = lib.mkNbPkgs { inherit (final) system; pkgs = final; };
       in removeAttrs nbPkgs [ "pinned" "nixops19_09" "krops" ];
 
       nixosModules = {
         # Uses the default system pkgs for nix-bitcoin.pkgs
         withSystemPkgs =  { pkgs, ... }: {
           imports = [ ./modules/modules.nix ];
-          nix-bitcoin.pkgs = (mkNbPkgs { inherit (pkgs) system; inherit pkgs; }).modulesPkgs;
+          nix-bitcoin.pkgs = (lib.mkNbPkgs { inherit (pkgs) system; inherit pkgs; }).modulesPkgs;
         };
 
         # Uses the nixpkgs version locked by this flake for nix-bitcoin.pkgs.
@@ -38,7 +40,7 @@
         # locked and the system nixpkgs versions differ.
         withLockedPkgs =  { config, ... }: {
           imports = [ ./modules/modules.nix ];
-          nix-bitcoin.pkgs = (mkNbPkgs { inherit (config.nixpkgs) system; }).modulesPkgs;
+          nix-bitcoin.pkgs = (lib.mkNbPkgs { inherit (config.nixpkgs) system; }).modulesPkgs;
         };
       };
 
@@ -51,6 +53,8 @@
       let
         pkgs = import nixpkgs { inherit system; };
 
+        nbPkgs = self.lib.mkNbPkgs { inherit system pkgs; };
+
         mkVMScript = vm: pkgs.writers.writeBash "run-vm" ''
           set -euo pipefail
           export TMPDIR=$(mktemp -d /tmp/nix-bitcoin-vm.XXX)
@@ -59,8 +63,6 @@
           QEMU_OPTS="-smp $(nproc) -m 1500" ${vm}/bin/run-*-vm
         '';
       in rec {
-        nbPkgs = self.mkNbPkgs { inherit system pkgs; };
-
         packages = flake-utils.lib.flattenTree (removeAttrs nbPkgs [
           "pinned" "modulesPkgs" "nixops19_09" "krops" "generate-secrets" "netns-exec"
         ]) // {
@@ -92,6 +94,11 @@
             }).vm;
         };
 
+        # Allow accessing the whole nested `nbPkgs` attrset (including `modulesPkgs`)
+        # via this flake.
+        # `packages` is not allowed to contain nested pkgs attrsets.
+        legacyPackages = { inherit nbPkgs; };
+
         defaultApp = apps.vm;
 
         apps = {