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.)

flake.lock

@@ -0,0 +1,60 @@
+{
+  "nodes": {
+    "flake-utils": {
+      "locked": {
+        "lastModified": 1623875721,
+        "narHash": "sha256-A8BU7bjS5GirpAUv4QA+QnJ4CceLHkcXdRp4xITDB0s=",
+        "owner": "numtide",
+        "repo": "flake-utils",
+        "rev": "f7e004a55b120c02ecb6219596820fcd32ca8772",
+        "type": "github"
+      },
+      "original": {
+        "owner": "numtide",
+        "repo": "flake-utils",
+        "type": "github"
+      }
+    },
+    "nixpkgs": {
+      "locked": {
+        "lastModified": 1628865210,
+        "narHash": "sha256-dB3IA8AYUQDXH+Xy/6nbv4QpIbVl88DphbcxJSUYiX4=",
+        "owner": "NixOS",
+        "repo": "nixpkgs",
+        "rev": "a445f5829889959d65ad65e5c961d5c67e1cd677",
+        "type": "github"
+      },
+      "original": {
+        "owner": "NixOS",
+        "ref": "nixos-21.05",
+        "repo": "nixpkgs",
+        "type": "github"
+      }
+    },
+    "nixpkgsUnstable": {
+      "locked": {
+        "lastModified": 1628779307,
+        "narHash": "sha256-ErivbgE5SGvNdFpq+Q50lw1JGGmvxBR/d71aW41S+1A=",
+        "owner": "NixOS",
+        "repo": "nixpkgs",
+        "rev": "4138cbd913fad85073e59007710e3f083d0eb7c6",
+        "type": "github"
+      },
+      "original": {
+        "owner": "NixOS",
+        "ref": "nixpkgs-unstable",
+        "repo": "nixpkgs",
+        "type": "github"
+      }
+    },
+    "root": {
+      "inputs": {
+        "flake-utils": "flake-utils",
+        "nixpkgs": "nixpkgs",
+        "nixpkgsUnstable": "nixpkgsUnstable"
+      }
+    }
+  },
+  "root": "root",
+  "version": 7
+}