add flake support

This change is fully backwards compatible.

We continue to use the standard non-flake evaluation mode in our
examples and internal tooling until the flakes design has stabilized.

'clightning-plugins = pkgs.recurseIntoAttrs' in pkgs/default.nix is
needed by flake-utils.lib.flattenTree in flake.nix.
It transforms the packages in `clightning-plugins` to top-level packages
named like `clightning-plugins/summary`. (The flake attr `packages`
must be a non-nested attrset of derivations.)

pkgs/default.nix

@@ -14,7 +14,7 @@ let self = {
   krops = import ./krops { };
   netns-exec = pkgs.callPackage ./netns-exec { };
   extra-container = pkgs.callPackage ./extra-container { };
-  clightning-plugins = import ./clightning-plugins pkgs self.nbPython3Packages;
+  clightning-plugins = pkgs.recurseIntoAttrs (import ./clightning-plugins pkgs self.nbPython3Packages);
   clboss = pkgs.callPackage ./clboss { };
   secp256k1 = pkgs.callPackage ./secp256k1 { };