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.

third_party/lisp/cl-colors.nix

@@ -1,6 +1,6 @@
 {
   depot ? import <depot> {},
-  universe ? import <universe> {},
+  briefcase ? import <briefcase> {},
   ...
 }:
 
@@ -13,7 +13,7 @@ in depot.nix.buildLisp.library {
   name = "cl-colors";
   deps = [
     depot.third_party.lisp.alexandria
-    universe.third_party.lisp.let-plus
+    briefcase.third_party.lisp.let-plus
   ];
   srcs = [
     "${src}/package.lisp"

third_party/lisp/let-plus.nix

@@ -1,6 +1,6 @@
 {
   depot ? import <depot> {},
-  universe ? import <universe> {},
+  briefcase ? import <briefcase> {},
   ...
 }:
 
@@ -13,7 +13,7 @@ in depot.nix.buildLisp.library {
   name = "let-plus";
   deps = [
     depot.third_party.lisp.alexandria
-    universe.third_party.lisp.anaphora
+    briefcase.third_party.lisp.anaphora
   ];
   srcs = [
     "${src}/package.lisp"

third_party/lisp/prove.nix

@@ -1,6 +1,6 @@
 {
   depot ? import <depot> {},
-  universe? import <universe> {},
+  briefcase ? import <briefcase> {},
   ...
 }:
 
@@ -16,7 +16,7 @@ in depot.nix.buildLisp.library {
     depot.third_party.lisp.cl-ansi-text
     depot.third_party.lisp.alexandria
     depot.third_party.lisp.uiop
-    universe.third_party.lisp.cl-colors
+    briefcase.third_party.lisp.cl-colors
   ];
   srcs = [
     "${src}/src/asdf.lisp"