feat: enable recursive library paths

Fix #839

citar-file.el

@@ -80,6 +80,7 @@ separator that does not otherwise occur in citation keys."
 
 (defvar citar-notes-paths)
 (defvar citar-library-paths)
+(defvar citar-library-paths-recursive)
 (defvar citar-library-file-extensions)
 (defvar citar-note-format-function)
 
@@ -177,6 +178,14 @@ whether entries have associated files."
   (when citar-file-variable
     (lambda (citekey) (and (citar-get-value citar-file-variable citekey) t))))
 
+(defun citar--library-dirs ()
+  "Return all directories to be searched for library files."
+  (mapcar (lambda (dir)
+            (cons dir (when citar-library-paths-recursive
+                        (directory-files-recursively dir "" :include-directories
+                                                     #'file-directory-p))))
+          citar-library-paths))
+
 (defun citar-file--get-from-file-field (&optional keys)
   "Return files for KEYS by parsing the `citar-file-variable' field.
 
@@ -191,7 +200,7 @@ files associated with KEYS."
   (when-let ((filefield citar-file-variable))
     (citar--check-configuration 'citar-library-paths 'citar-library-file-extensions
                                 'citar-file-parser-functions)
-    (let ((dirs (append citar-library-paths
+    (let ((dirs (append (citar--library-dirs)
                         (mapcar #'file-name-directory (citar--bibliography-files)))))
       (citar--get-resources-using-function
        (lambda (citekey entry)

citar.el

@@ -91,6 +91,11 @@ buffer.")
   :group 'citar
   :type '(repeat directory))
 
+(defcustom citar-library-paths-recursive nil
+  "Whether library paths should be searched recursively."
+  :group 'citar
+  :type 'boolean)
+
 (defcustom citar-library-file-extensions nil
   "List of file extensions to filter for related files.