feat(modules): use package set for port sources

[getchoo]

Closes #240

The main goal here is to avoid discrepancies with the versions of Nixpkgs we have tested on and those on our users' systems -- i.e., us falling out of sync or moving too quickly with Nixpkgs will no longer cause problems in our modules

As a side effect of these changes, we also see some other improvements:

• No more builtin fetchers
  • Builtin fetchers have a large effect on evaluation by blocking it to download files -- similar to IFD. This is why they are not allowed in Nixpkgs.
  • Using Nixpkgs' fetchers allows us to download most sources at build time, considerably improving evaluation performance for those who use many of our modules
• Less space taken up by (most) sources
  • Done by only packaging what's needed by the module
  • This should make interacting with sources much easier
  • It also allows us to easily share modifications to ports across home-manager and NixOS
• Fewer rebuilds
  • Some derivations brought over from Nixpkgs have been modified to build and install all variants instead of only one set. While this slightly increases storage use, it avoids rebuilds when you change your accent, flavor, etc options

[isabelroses]

might be worth adding a default meta.maintainers to prevent log spam for users that have nixpkgs.config.showDerivationWarnings = [ "maintainerless" ];