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/lockfile.hh

@@ -81,5 +81,7 @@ struct LockFile : LockedInputs
 
 std::ostream & operator <<(std::ostream & stream, const LockFile & lockFile);
 
+InputPath parseInputPath(std::string_view s);
+
 }