This will make it easier to change it if we want to decouple from pkgs.

It does not make sense to look for derivations within derivations,
not even when `recurseForDerivations` is true. Nix does not do that either:

https://github.com/NixOS/nix/blob/ebc024df2287085d48ed6194aa756fd70c07f76c/src/libexpr/get-drvs.cc#L346-L355

The `packagesWith` function expected an attrSet but `packagesWithUpdateScript`
could be passing it a derivation or a list when the attribute path
supplied by user through the `--argstr path` argument pointed to one.
It only worked because derivations are also attrSets and contain their
outputs as attributes, and did not work for lists at all.

Additionally, the improper handling would cause the `src` attribute
to be built in some rare cases (`mkYarnPackage` seems to trigger this).

Rewriting the `packagesWith` function to be inductive with a derivation
as a base case and attrSets and lists as inductive steps is much cleaner
and also fixes the unnecessary build.