Make flake input fetching lazy

As long as the flake input is locked, it is now only fetched when it
is evaluated (e.g. "nixpkgs" is fetched when
"inputs.nixpkgs.<something>" is evaluated).

This required adding an "id" attribute to the members of "inputs" in
lockfiles, e.g.

  "inputs": {
    "nixpkgs/release-19.03": {
      "id": "nixpkgs",
      "inputs": {},
      "narHash": "sha256-eYtxncIMFVmOHaHBtTdPGcs/AnJqKqA6tHCm0UmPYQU=",
      "nonFlakeInputs": {},
      "uri": "github:edolstra/nixpkgs/e9d5882bb861dc48f8d46960e7c820efdbe8f9c1"
    }
  }

because the flake ID needs to be known beforehand to construct the
"inputs" attrset.

Fixes #2913.

src/nlohmann/json_fwd.hpp

@@ -0,0 +1,10 @@
+#pragma once
+
+namespace nlohmann {
+
+struct json : basic_json<>
+{
+    using basic_json<>::basic_json;
+};
+
+}