Add --update-input flag to update a specific flake input

Typical usage:

  $ nix flake update ~/Misc/eelco-configurations/hagbard --update-input nixpkgs

to update the 'nixpkgs' input of a flake while leaving every other
input unchanged.

The argument is an input path, so you can do e.g. '--update-input
dwarffs/nixpkgs' to update an input of an input.

Fixes #2928.

src/libexpr/flake/lockfile.hh

@@ -27,6 +27,8 @@ struct LockedInputs
     bool isImmutable() const;
 
     std::optional<LockedInput *> findInput(const InputPath & path);
+
+    void removeInput(const InputPath & path);
 };
 
 /* Lock file information about a flake input. */