lang/org: lazy-load babel libraries

Removes +org-babel-languages and no longer eagerly loads babel
libraries. If an ob-*.el exists for the language, it will be loaded once
you execute its src block.

Warning: this may interfere with tangling. An unloaded library can't
register a language extension in org-babel-tangle-lang-exts (if any).
This means babel won't be able to figure out the correct file extension
for certain src blocks.

Either load the package explicitly or provide a filename + extension for
the TARGET-FILE argument:

  (require 'ob-rust)
  (org-babel-tangle-file "notes.org")
  ;; or
  (org-babel-tangle-file "notes.org" "notes.rs")

modules/lang/org/+babel.el

@@ -2,29 +2,6 @@
 
 (add-hook 'org-load-hook #'+org|init-babel)
 
-(defvar +org-babel-languages
-  '(calc
-    css
-    emacs-lisp
-    haskell
-    js
-    latex
-    ledger
-    lilypond
-    lisp
-    matlab
-    plantuml
-    python
-    restclient ; ob-restclient
-    ruby
-    rust       ; ob-rust
-    shell
-    sqlite
-    sql-mode   ; ob-sql-mode
-    translate) ; ob-translate
-  "A list of org-babel languages to load.")
-
-
 (defun +org|init-babel ()
   (setq org-src-fontify-natively t      ; make code pretty
         org-src-preserve-indentation t  ; use native major-mode indentation
@@ -32,10 +9,15 @@
         org-src-window-setup 'current-window
         org-confirm-babel-evaluate nil) ; you don't need my permission
 
-  (org-babel-do-load-languages
-   'org-babel-load-languages
-   (cl-loop for sym in +org-babel-languages
-            collect (cons sym t)))
+  (defun +org*babel-execute-src-block (orig-fn &rest args)
+    "Load babel libraries as needed when babel blocks are executed."
+    (let* ((language (org-element-property :language (org-element-at-point)))
+           (lang-sym (intern language)))
+      (unless (cdr (assoc lang-sym org-babel-load-languages))
+        (require (intern (format "ob-%s" language)))
+        (add-to-list 'org-babel-load-languages (cons lang-sym t)))
+      (apply orig-fn args)))
+  (advice-add #'org-babel-execute-src-block :around #'+org*babel-execute-src-block)
 
   ;; I prefer C-c C-c for confirming over the default C-c '
   (map! :map org-src-mode-map "C-c C-c" #'org-edit-src-exit)