Represent 'follows' inputs explicitly in the lock file

This fixes an issue where lockfile generation was not idempotent:
after updating a lockfile, a "follows" node would end up pointing to a
new copy of the node, rather than to the original node.

src/libexpr/flake/call-flake.nix

@@ -8,14 +8,41 @@ let
     builtins.mapAttrs
       (key: node:
         let
+
           sourceInfo =
             if key == lockFile.root
             then rootSrc
             else fetchTree (node.info or {} // removeAttrs node.locked ["dir"]);
+
           subdir = if key == lockFile.root then rootSubdir else node.locked.dir or "";
+
           flake = import (sourceInfo + (if subdir != "" then "/" else "") + subdir + "/flake.nix");
-          inputs = builtins.mapAttrs (inputName: key: allNodes.${key}) (node.inputs or {});
+
+          inputs = builtins.mapAttrs
+            (inputName: inputSpec: allNodes.${resolveInput inputSpec})
+            (node.inputs or {});
+
+          # Resolve a input spec into a node name. An input spec is
+          # either a node name, or a 'follows' path from the root
+          # node.
+          resolveInput = inputSpec:
+              if builtins.isList inputSpec
+              then getInputByPath lockFile.root inputSpec
+              else inputSpec;
+
+          # Follow an input path (e.g. ["dwarffs" "nixpkgs"]) from the
+          # root node, returning the final node.
+          getInputByPath = nodeName: path:
+            if path == []
+            then nodeName
+            else
+              getInputByPath
+                # Since this could be a 'follows' input, call resolveInput.
+                (resolveInput lockFile.nodes.${nodeName}.inputs.${builtins.head path})
+                (builtins.tail path);
+
           outputs = flake.outputs (inputs // { self = result; });
+
           result = outputs // sourceInfo // { inherit inputs; inherit outputs; inherit sourceInfo; };
         in
           if node.flake or true then