Replace calls to (getenv "BRIEFCASE") with constants/briefcase

I would prefer to define constants/briefcase in terms of `(getenv "BRIEFCASE")`
and assert that `(f-exists? (getenv "BRIEFCASE"))`, in one location:
constants.el

emacs/.emacs.d/wpc/bookmark.el

@@ -21,6 +21,7 @@
 (require 'list)
 (require 'string)
 (require 'set)
+(require 'constants)
 
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 ;; Constants
@@ -60,7 +61,7 @@ Otherwise, open with `counsel-find-file'."
                   :path "~/depot"
                   :kbd "t")
    (make-bookmark :label "briefcase"
-                  :path (getenv "BRIEFCASE")
+                  :path constants/briefcase
                   :kbd "b")
    (make-bookmark :label "current project"
                   :path constants/current-project

emacs/.emacs.d/wpc/constants.el

@@ -13,19 +13,24 @@
 (require 'prelude)
 (require 'f)
 
+(prelude/assert (f-exists? (getenv "BRIEFCASE")))
+
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 ;; Configuration
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 
+(defconst constants/briefcase
+  (getenv "BRIEFCASE")
+  "Path to my monorepo, which various parts of my configuration rely on.")
+
 ;; TODO: Consider merging `ui.el' and `misc.el' because those are the only
 ;; current consumers of these constants, and I'm unsure if the indirection that
 ;; globally defined constants introduces is worth it.
 
-(defconst constants/current-project "~/briefcase"
+(defconst constants/current-project
+  constants/briefcase
   "Variable holding the directory for my currently active project.")
 
-(prelude/assert (f-directory? constants/current-project))
-
 (defconst constants/mouse-kbds
   '([mouse-1] [down-mouse-1] [drag-mouse-1] [double-mouse-1] [triple-mouse-1]
     [mouse-2] [down-mouse-2] [drag-mouse-2] [double-mouse-2] [triple-mouse-2]

emacs/.emacs.d/wpc/wpc-misc.el

@@ -13,6 +13,7 @@
 (require 'project)
 (require 'f)
 (require 'dash)
+(require 'constants)
 
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 ;; Configuration
@@ -180,10 +181,11 @@
 ;; trim whitespace on save
 (add-hook 'before-save-hook #'delete-trailing-whitespace)
 
-;; call `git secret hide` after saving ~/briefcase/secrets.json
+;; call `git secret hide` after saving secrets.json
 (add-hook 'after-save-hook
           (lambda ()
-            (when (f-equal? (buffer-file-name) "~/briefcase/secrets.json")
+            (when (f-equal? (buffer-file-name)
+                            (f-join constants/briefcase "secrets.json"))
               (shell-command "git secret hide"))))
 
 ;; use tabs instead of spaces
@@ -207,7 +209,7 @@
 (defun project-find-function--briefcase (dir)
   "Find the nearest default.nix file; otherwise, terminate at the .git
   directory."
-  (when (s-starts-with? (getenv "BRIEFCASE") (f-expand dir))
+  (when (s-starts-with? constants/briefcase (f-expand dir))
     (if (f-exists? (f-join dir "default.nix"))
         (cons 'transient dir)
       (project-find-function--briefcase (f-parent dir)))))

emacs/.emacs.d/wpc/wpc-nix.el

@@ -9,8 +9,7 @@
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 
 (require 'device)
-
+(require 'constants)
-(prelude/assert (f-exists? (getenv "BRIEFCASE")))
 
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 ;; Library
@@ -28,7 +27,7 @@
          (bname (format "*%s*" pname)))
     (start-process pname bname
                    "nix-env"
-                   "-I" (format "briefcase=%s" (getenv "BRIEFCASE"))
+                   "-I" (format "briefcase=%s" constants/briefcase)
                    "-f" "<briefcase>" "-iA" emacs)
     (display-buffer bname)))