Add position info to path values

(Actually, this adds a position field to *all* values.)

This allows improving the "inefficient double copy" warning by showing
where the source path came from in the source, e.g.

  warning: Performing inefficient double copy of path '/home/eelco/Dev/patchelf/' to the store at /home/eelco/Dev/patchelf/flake.nix:30:17. This can typically be avoided by rewriting an attribute like `src = ./.` to `src = builtins.path { path = ./.; name = "source"; }`.

src/libutil/include/nix/util/pos-idx.hh

@@ -15,12 +15,12 @@ class PosIdx
 private:
     uint32_t id;
 
+public:
     explicit PosIdx(uint32_t id)
         : id(id)
     {
     }
 
-public:
     PosIdx()
         : id(0)
     {
@@ -45,6 +45,11 @@ public:
     {
         return std::hash<uint32_t>{}(id);
     }
+
+    uint32_t get() const
+    {
+        return id;
+    }
 };
 
 inline PosIdx noPos = {};