Support building wpcarros-emacs

I'm using a Makefile until I can remember the command:

```shell
> nix-env -f . -i
```

This will install (i.e. `-i`) any derivations instantiated from the Nix
expression resolvable by `-f`. Ideally the incantation will look something like
this:

```shell
> nix-env -f '<universe>' -iA emacs
```

Informing `nix-env` to install all of the derivations created by the expression
at attribute `emacs` in my `<universe>` repository. For now two things are
preventing this:

1. `emacs` isn't an attribute in my top-level expression defined in the
   `default.nix`.
2. If I do add `emacs` as an attribute and call the above command, my usage of
   `readTree` results in `pkgs` missing `.lib` and a few other stdlib commands
   that are available in `(import <nixpkgs> {})`.

A fix for both of these should be forthcoming.

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