We previously allowed you to map any flake URL to any other flake URL,
including shorthand flakerefs, indirect flake URLs like `flake:nixpkgs`,
direct flake URLs like `github:NixOS/nixpkgs`, or local paths.
But flake registry entries mapping from direct flake URLs often come
from swapping the 'from' and 'to' arguments by accident, and even when
created intentionally, they may not actually work correctly.
This patch rejects those URLs (and fully-qualified flake: URLs), making
it harder to swap the arguments by accident.
Fixes#181.
Change-Id: I24713643a534166c052719b8770a4edfcfdb8cf3
