Inconsistent override mechanisms

[infinisil]

Issue description

There's too many ways to override packages and they're confusing users. This includes overlays, .override, .overrideAttrs, .extend, .overrideScope', overrideDerivation, packageOverrides and probably more, each of which is needed for very specific use cases. Knowing which one is the right one to use is very hard and typically requires reading the nixpkgs source code.

Goal

This issue is done when users aren't confused anymore about overrides.

[nbp]

All override mechanism can be replaced by the update operator //, if we were to move stdenv.mkDerivation to be applied after the fix-point of Nixpkgs.

Making override would be easy within Nixpkgs or within an overlay. On the other hand, these functions would still be required for everybody who want to externally extend Nixpkgs. In which case, it might be preferable to highly recommend overlays as the only mean of extending Nixpkgs with custom packages, rather than extending Nixpkgs externally.

See also NixOS/rfcs#3

[aameen-tulip]

This was one I've been working on a lot.
This is my prototype, certainly not perfect but I've been refining it a lot over the last month.
It's comprised of two sorts of recursive attrsets, one for an "entry" ( single package or element ) and entry-sets ( like a package-set or scope ). They share a common base ExtInfo that is an extensible attrset full of functors.
ExtInfo: https://github.com/aameen-tulip/at-node-nix/blob/292ba02b88e3251e3828c257c1c1ce5dd231fe90/lib/meta.nix#L148
metaEnt: https://github.com/aameen-tulip/at-node-nix/blob/292ba02b88e3251e3828c257c1c1ce5dd231fe90/lib/meta.nix#L390
metaSet: https://github.com/aameen-tulip/at-node-nix/blob/292ba02b88e3251e3828c257c1c1ce5dd231fe90/lib/meta.nix#L478

These are used for my Node.js routines that dynamically generate derivations from various types of Node.js metadata ( package[-lock].json, manifest.json, etc ).
https://github.com/aameen-tulip/at-node-nix/blob/224bdf7de6556e36f58b351699062726bf6298bf/pkgs/node-pkg-set.nix#L151 ( this is a branch with a slightly older definition of the "meta" constructs; but it illustrates the usage ).

[roberth]

Possible approach: #273815

[nixos-discourse]

This issue has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/is-this-the-current-state-of-things/45944/13

[nixos-discourse]

This issue has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/override-derivation-with-overrideattrs-any-way-to-handle-let-in/47307/2