Before this change, it was not possible to use string paths, because then the `types.str.check` would succeed. So the only paths that could be used were ones from the local filesystem via e.g. `./some/path`. We can easily fix this by using `types.path.check` instead to determine whether we are dealing with a path. This notably also allows using Nix-fetched sources as the content, e.g. `fetchFromGitHub { ... } + "/some/file"`