@brief Parses a flake URL into a (key, url) tuple.
@details
Example: “github:owner/repo” -> (“repo”, “github:owner/repo”)
Example: “github:owner/repo~branch” -> (“repo”, “github:owner/repo~branch”)
@param url The flake URL to parse.
@return A tuple containing the key and the original URL.
@brief Parses a flake URL into a package string.
@details
Example: “github:owner/repo” -> “repo.packages.${system}.default”
@param url The flake URL to parse.
@return The package string following the standard convention for flake packages.