fetchers should be overridable

[lilyball]

Describe the bug
None of the fetchers that I've looked at so far support overriding the fetch arguments. This is rather annoying when trying to override derivations that use them, as I need to repeat the original fetcher in my override instead of merely overriding the arguments I want to change.

This could be solved by wrapping all of the fetchers with lib.makeOverridable. I don't know what the performance implications are of turning these functions into functors instead, but I would hope it's negligible.

I was going to simply submit a PR for this, but we have a lot more fetchers than I thought, and I didn't want to do that work unless I knew it would be accepted.

As an example, if I want to change the version of ffsend, right now I have to write

ffsend.overrideAttrs (super: rec {
  version = "0.2.64";
  cargoSha256 = "…";
  src = fetchFromGitLab {
    owner = "timvisee";
    repo = "ffsend";
    rev = "v${version}";
    sha256 = "…";
  };
})

This is repeating stuff that I really shouldn't have to do here, including the fact that ffsend uses GitLab instead of GitHub, and the repo/owner. I should be able to instead write

ffsend.overrideAttrs (super: rec {
  version = "0.2.64";
  cargoSha256 = "…";
  src = super.src.override {
    rev = "v${version}";
    sha256 = "…";
  };
})

And with the mkDerivation changes proposed in #94198 that would shrink even further to

ffsend.overrideAttrs (super: {
  version = "0.2.64";
  cargoSha256 = "…";
  src = super.src.override {
    sha256 = "…";
  };
})

[stale]

I marked this as stale due to inactivity. → More info

[Atemu]

Still important to me.