POC of overlays approach to customization

[michaelpj]

This is a draft implementation of the idea I put forward in #134.

Using this I was able to successfully:

• Get nixpkgs from an alternate source
  • I removed nixpkgs sources.json and unset NIX_PATH
  • I added an overlay which set nixpkgs = ...
  • Other sources using e.g. type: "tarball" continued to work.
• Add a new fetcher type
  • I added an overlay setting __fetchers = super.__fetchers // { fetchgit = (import self.nixpkgs {}).fetchgit; };
  • Other sources with type: "fetchgit" worked as expected.

Ideally I'd add tests for these usecases but I'm too lazy to do it for the POC.

Downsides:

• The way of importing sources.nix changes.
• The nixpkgs entry in sources is less special, so needs to be imported with builtin-tarball so that the "bootstrapping" works. Arguably this is an upside!
• You actually need to add arguments to get the fetchgit example to work: this is solved in newer nixpkgs, and we can probably get rid of the arguments for the fetchzip and fetchurl calls too.
  • (Previously, makeOverridable didn't preserve function arguments, now it does. In 19.09 but not 19.03.)

I think the downsides are all fine with a tiny bit of documentation, since this is advanced usage anyway.

[michaelpj]

I pushed my two examples in niv itself (obviously do not merge with those there!).

Some thoughts:

• Maybe sources should go in the overlay as __sources or something.
  • This would allow changing the location of sources.json by simply setting it to something different.
  • It doesn't help with people who want to move sources.nix.
• People can add fetchers this way but it's unclear how it will interact with updating.