Don't hardcode the function used to fetch the source

[P-E-Meunier]

Currently, bundix hardcodes the way users can download sources, i.e. the produced files look like:

    source = {
      remotes = ["https://rubygems.org"];
      sha256 = "0niqzvclkb6205vdc4cx8gz2yjy5csy3w8cm15rgr1yqwigh1bqn";
      type = "gem";
    };

Which makes it hard to try and compile local versions, and possibly patch the sources, or report upstream bugs. If it were of the following form, this would be easy:

    source = fetchgem {
      remotes = ["https://rubygems.org"];
      sha256 = "0niqzvclkb6205vdc4cx8gz2yjy5csy3w8cm15rgr1yqwigh1bqn";
    };

With fetchgem a function doing exactly the same as it does today, but replaceable with fetchurl or even source = ./;.