refactor(mu4e): replace +mu4e-backend w/ +offlineimap/+mbsync flags

`+mu4e-backend` is now deprecated and will be removed in v3.0.

modules/email/mu4e/README.org

@@ -37,6 +37,10 @@ This module makes Emacs an email client, using [[https://www.djcbsoftware.nl/cod
 - +gmail ::
   Enable gmail-specific configuration for mail ~To~ or ~From~ a gmail address,
   or a maildir with ~gmail~ in the name.
+- +mbsync ::
+  Use [[https://isync.sourceforge.io/][mbsync]] to synchronize with mu.
+- +offlineimap ::
+  Use [[https://www.offlineimap.org/about/][offlineimap]] to synchronize with mu.
 - +org ::
   Use [[doom-package:org-msg]] for composing email in Org, then sending a multipart text (ASCII
   export) and HTML message.
@@ -124,34 +128,30 @@ programs.emacs = {
 #+end_quote
 
 ** offlineimap
-This module uses =mbsync= by default. To use =offlineimap=, change
-~+mu4e-backend~:
-#+begin_src emacs-lisp
-(setq +mu4e-backend 'offlineimap)
-#+end_src
+Enable this module with the =+offlineimap= flag, then [[https://www.offlineimap.org/doc/quick_start.html][write a configuration file]]
+for =offlineimap= ([[https://github.com/OfflineIMAP/offlineimap/blob/master/offlineimap.conf][a sample config file can be found in offlineimap's repo]]).
 
-Next, you need to write a configuration file for =offlineimap=. Mine can be
-found [[https://github.com/hlissner/dotfiles/blob/be0dce5dae8f3cbafaac0cc44269d84b4a742c46/shell/mu/][in my dotfiles repository]]. It is configured to download mail to
-~\~/.mail~. I use [[https://www.passwordstore.org/][unix pass]] to securely store my login credentials. You can find
-a *very* detailed configuration [[https://github.com/OfflineIMAP/offlineimap/blob/master/offlineimap.conf][here]].
+Next, download your email with ~$ offlineimap -o~. This may take a while,
+depending on how many emails need downloading.
 
-Next you can download your email with ~$ offlineimap -o~. This may take a while,
-especially if you have thousands of mails.
-
-You can now proceed with the [[#mu-and-mu4e][mu and mu4e]] section.
+Then proceed to the "mu and mu4e" section below.
 
 ** mbsync
-The steps needed to set up =mu4e= with =mbsync= are very similar to the ones for
-[[#offlineimap][offlineimap]].
+After this module has been enabled with the =+mbsync= flag, the steps to set up
+=mbsync= are similar to the ones for offlineimap:
 
-Start with writing a =~/.mbsyncrc=. An example for Gmail can be found on
-[[http://pragmaticemacs.com/emacs/migrating-from-offlineimap-to-mbsync-for-mu4e/][pragmaticemacs.com]]. A non-Gmail example is available as a gist [[https://gist.github.com/agraul/60977cc497c3aec44e10591f94f49ef0][here]]. The [[http://isync.sourceforge.net/mbsync.html][manual
-page]] contains all needed information to set up your own.
+Start with writing a =~/.mbsyncrc=. Here are some examples:
 
-Next you can download your email with ~$ mbsync --all~. This may take a while,
-but should be quicker than =offlineimap= ;).
+- [[https://pragmaticemacs.wordpress.com/2016/03/22/migrating-from-offlineimap-to-mbsync-for-mu4e/][For Gmail accounts]],
+- [[https://rakhim.org/fastmail-setup-with-emacs-mu4e-and-mbsync-on-macos/][For Fastmail accounts]],
+- [[https://gist.github.com/agraul/60977cc497c3aec44e10591f94f49ef0][A more generic example for other services]].
 
-You can now proceed with the [[#mu-and-mu4e][mu and mu4e]] section.
+The [[http://isync.sourceforge.net/mbsync.html][manual page]] contains all needed information to set up your own.
+
+Next, download your email with ~$ mbsync --all~. This may take a while, but
+should be quicker than =offlineimap= ;).
+
+Then proceed to the "mu and mu4e" section below.
 
 *** Faster syncing
 It's possible to use IMAP IDLE to be quickly notified of updates, then use a
@@ -161,11 +161,9 @@ If this is of interest, this approach can be seen [[https://tecosaur.github.io/e
 [[https://gitlab.com/shackra/goimapnotify][goimapnotify]] is used for this.
 
 ** Fetching your mail in other ways
-You also have the possiblity to invoke an arbitary shell command to fetch your
-mail by disabling this module's backend selection and setting the value of the
-~mu4e-get-mail-command~ to the command you want to execute:
+Mu4e can be configured to call an arbitary shell command to fetch your email. To
+use it, set [[var:mu4e-get-mail-command]]:
 #+begin_src emacs-lisp
-(setq +mu4e-backend nil)
 (after! mu4e
   (setq mu4e-get-mail-command "your_command"))
 #+end_src

modules/email/mu4e/config.el

@@ -2,6 +2,7 @@
 
 (defvar +mu4e-backend 'mbsync
   "Which backend to use. Can either be offlineimap, mbsync or nil (manual).")
+(make-obsolete-variable '+mu4e-backend "Use the :email mu4e module's +mbsync or +offlineimap flags instead" "3.0.0")
 
 (defvar +mu4e-personal-addresses 'nil
   "Alternative to mu4e-personal-addresses that can be set for each account (mu4e context).")
@@ -23,6 +24,14 @@
         (lambda (&rest _)
           (expand-file-name ".attachments" (mu4e-root-maildir))))
   :config
+  (cond ((or (modulep! +mbsync)
+             (eq +mu4e-backend 'mbsync))
+         (setq mu4e-get-mail-command "mbsync -a"
+               mu4e-change-filenames-when-moving t))
+        ((or (modulep! +offlineimap)
+             (eq +mu4e-backend 'offlineimap))
+         (setq mu4e-get-mail-command "offlineimap -o -q")))
+
   (when (version< mu4e-mu-version "1.8")
     ;; Define aliases to maintain backwards compatibility. The list of suffixes
     ;; were obtained by comparing mu4e~ and mu4e-- functions in `obarray'.
@@ -67,13 +76,6 @@ is non-nil."
           mu4e-view-image-max-width 800
           mu4e-view-use-gnus t))
 
-  (pcase +mu4e-backend
-    (`mbsync
-     (setq mu4e-get-mail-command "mbsync -a"
-           mu4e-change-filenames-when-moving t))
-    (`offlineimap
-     (setq mu4e-get-mail-command "offlineimap -o -q")))
-
   (setq mu4e-update-interval nil
         mu4e-notification-support t
         mu4e-sent-messages-behavior 'sent