Refactor nodeLibs/default.nix to be more callPackage friendly

[ixmatus]

The nodeLibs/default.nix can be refactored such that it can be callPackage'ed and the generated nodePackage set be the direct result of doing so; meaning, we can remove the makeOverridable on the generatePackages function because callPackage will do that for us.

This also entails moving the arguments for overrides and other things out to the top intake arguments that callPackage would fill-in so that when a user uses the .override { ... } on the result of callPackage'ing nodeLibs/default.nix they can provide the overrides = ... attribute.

This would remove the indirection that currently exists. It is also a backwards breaking change.