Make a string into a valid Nix derivation name

drvname.nix

{ lib }: rec {
  isValidDrvNameChar = c: "a" <= c && c <= "z" || "A" <= c && c <= "Z" || "0" <= c && c <= "9" || c == "+" || c == "-" || c == "_" || c == "?" || c == "=" || c == ".";

  makeValidDrvName = { str, replacement ? "?" }:
    let
      newName = lib.stringAsChars (c: if isValidDrvNameChar c then c else replacement) str;
    in if builtins.substring 0 1 newName == "." then "_" + newName else newName;
}