Add flag --override-input to override specific lock file entries

E.g.

  $ nix flake update ~/Misc/eelco-configurations/hagbard \
    --override-input 'dwarffs/nixpkgs' ../my-nixpkgs

overrides the 'nixpkgs' input of the 'dwarffs' input of the top-level
flake.

Fixes #2837.

src/libexpr/flake/flake.hh

@@ -59,7 +59,16 @@ struct LockedFlake
     Fingerprint getFingerprint() const;
 };
 
-LockedFlake lockFlake(EvalState &, const FlakeRef &, LockFileMode);
+struct LockFlags
+{
+    std::map<InputPath, FlakeRef> inputOverrides;
+};
+
+LockedFlake lockFlake(
+    EvalState &,
+    const FlakeRef &,
+    LockFileMode,
+    const LockFlags &);
 
 void callFlake(EvalState & state,
     const Flake & flake,