pkgs: import pinned nixpkgs in default.nix

pkgs/default.nix now explicitly specifies all its dependencies as arguments.
This is required for flake support.

Also simplify pinned.nix and python-packages by removing unused attrs.

pkgs/default.nix

@@ -1,4 +1,10 @@
-{ pkgs ? import <nixpkgs> {} }:
+let
+  nixpkgsPinned = import ./nixpkgs-pinned.nix;
+in
+# Set default values for use without flakes
+{ pkgs ? import <nixpkgs> { config = {}; overlays = []; }
+, pkgsUnstable ? import nixpkgsPinned.nixpkgs-unstable { config = {}; overlays = []; }
+}:
 let self = {
   spark-wallet = pkgs.callPackage ./spark-wallet { };
   liquid-swap = pkgs.python3Packages.callPackage ./liquid-swap { };
@@ -16,7 +22,7 @@ let self = {
     packageOverrides = import ./python-packages self;
   }).pkgs;
 
-  pinned = import ./pinned.nix;
+  pinned = import ./pinned.nix pkgs pkgsUnstable;
 
   modulesPkgs = self // self.pinned;
 }; in self