emacs: get packages from local emacs

[hraban]

Description

This fixes a problem I had locally where pinning an Emacs from a previous nixpkgs version in my config didn't also extend to pinning the packages.

I will admit I'm not 100% if I'm solving this the right way but it feels like this is what you'd want? And is this the right way to do so?

Whether this is backwards compatible depends on your interpretation of what the spec was in the first place; was this a bug or a feature?

Checklist

• Change is backwards compatible.

• Code formatted with ./format.

• Code tested through nix-shell --pure tests -A run.all or nix develop --ignore-environment .#all using Flakes.

• Test cases updated/added. See example.

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