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/flake.hh

@@ -19,9 +19,10 @@ typedef std::map<FlakeId, FlakeInput> FlakeInputs;
 
 struct FlakeInput
 {
-    FlakeRef ref;
+    std::optional<FlakeRef> ref;
     bool isFlake = true;
     std::optional<InputPath> follows;
+    bool absolute = false; // whether 'follows' is relative to the flake root
     FlakeInputs overrides;
 };