Rename dotfiles -> briefcase

Renaming my mono-repo briefcase.

I first introduced this commit in master, but it introduced a bug where one of
two things would happen:

1. Emacs wouldn't start and would crash X.
2. Emacs would start but my keyboard wouldn't work.

I learned some valuable debugging skills in the process. Here are some of them:

When my keyboard was broken, I wanted to control my computer using my
laptop. Thankfully this is possible by using `x2x`, which forward X events from
the SSH client to the SSH host.

```shell
> # I'm unsure if this is the *exact* command
> ssh -X desktop x2x -west :0.0
```

Git commit-local bisecting. I didn't need to do a `git bisect` because I knew
which commit introduced the bug; it was HEAD, master. But -- as you can see from
the size of this commit -- there are many changes involved. I wanted to binary
search through the changes, so I did the following workflow using `magit`:

- git reset --soft HEAD^
- git stash 1/2 of the files changed
- re-run `nix-env -f ~/briefcase/emacs -i`
- restart X session
- If the problem persists, the bug exists in the non-stashed files. Repeat the
  process until you find the bug.

In my case, the bug was pretty benign. Calling `(exwm/switch "Dotfiles")` at the
bottom of `window-manager.el` was failing because "Dotfiles" is the name of a
non-existent workspace; it should've been `(exwm/switch "Briefcase")`.

There may have been more problems. I changed a few other things along the way,
including exposing the env vars BRIEFCASE to `wpcarros-emacs` inside of
`emacs/default.nix`.

The important part is that this was a valuable learning opportunity, and I'm
glad that I'm walking away from the two days of "lost productivity" feeling
actually productive.

emacs/.emacs.d/wpc/dotfiles.el

@@ -20,9 +20,12 @@
 (defconst dotfiles/install-kbds? t
   "When t, install the keybindings.")
 
+(defconst dotfiles/directory (getenv "BRIEFCASE")
+  "The root directory of my configuration files.")
+
 (defconst dotfiles/whitelist
-  '(("compton" . "~/.config/compton.conf")
-    ("dotfiles" . "~/dotfiles/")
+  `(("compton" . "~/.config/compton.conf")
+    ("dotfiles" . ,dotfiles/directory)
     ("functions" . "~/functions.zsh")
     ("aliases" . "~/aliases.zsh")
     ("variables" . "~/variables.zsh")
@@ -47,7 +50,7 @@
 (defun dotfiles/find-emacs-file (name)
   "Call `find-file' on NAME located in dotfiles's emacs.d directory."
   (find-file
-   (f-join "~/dotfiles/configs/shared/.emacs.d" name)))
+   (f-join dotfiles/directory "emacs/.emacs.d" name)))
 
 (provide 'dotfiles)
 ;;; dotfiles.el ends here