feat(sterni/mblog): add package to read maildirs

Change-Id: I7be8d8b7d12bb194712aa26f3ddad74340357779
Reviewed-on: https://cl.tvl.fyi/c/depot/+/5070
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>

users/sterni/mblog/maildir.lisp

@@ -0,0 +1,17 @@
+(in-package :maildir)
+(declaim (optimize (safety 3)))
+
+(defun list (dir)
+  "Returns a list of pathnames to messages in a maildir. The messages are
+  returned in no guaranteed order. Note that this function doesn't fully
+  implement the behavior prescribed by maildir(5): It only looks at `cur`
+  and `new` and won't clean up `tmp` nor move files from `new` to `cur`,
+  since it is strictly read-only."
+  (flet ((subdir-contents (subdir)
+           (directory
+            (merge-pathnames
+             (make-pathname :directory `(:relative ,subdir)
+                            :name :wild :type :wild)
+             dir))))
+    (mapcan #'subdir-contents '("cur" "new"))))
+