refactor: cut down on pseudo-features

Way back, I added these three pseudo-features:

  (featurep 'dynamic-modules)
  (featurep 'harfbuzz)
  (featurep 'jansson)

Why? Because some build features have pseudo features (like
`tty-child-frames`, `pgtk`, and `threads`), but others don't, and I
wanted more consistency around build feature detection. Years later, I
realized it wasn't used much internally and only ended up confusing
readers who didn't realize these were Doom's additions and not built
into Emacs. Emacs' idiosyncrasies may not be nice or elegant, but
they're less surprising to elisp beginners and veterans alike.

lisp/doom.el

@@ -144,6 +144,7 @@
 (push :system features)
 (put :system 'subfeatures doom-system)
 
+;;; DEPRECATED: Remove in v3
 ;; Emacs needs a more consistent way to detect build features, and the docs
 ;; claim `system-configuration-features' is not da way. Some features (that
 ;; don't represent packages) can be found in `features' (which `featurep'

modules/term/vterm/config.el

@@ -1,7 +1,7 @@
 ;;; term/vterm/config.el -*- lexical-binding: t; -*-
 
 (use-package! vterm
-  :when (featurep 'dynamic-modules)
+  :when (bound-and-true-p module-file-suffix)  ; requires dynamic-modules support
   :commands vterm-mode
   :hook (vterm-mode . doom-mark-buffer-as-real-h)
   :hook (vterm-mode . hide-mode-line-mode) ; modeline serves no purpose in vterm

modules/tools/tree-sitter/doctor.el

@@ -1,6 +1,6 @@
 ;;; tools/tree-sitter/doctor.el -*- lexical-binding: t; -*-
 
-(unless (featurep 'dynamic-modules)
+(unless (bound-and-true-p module-file-suffix)  ; requires dynamic-modules support
   (error! "Emacs not built with dynamic modules support"))
 
 (if (version< emacs-version "29.1")

modules/ui/ligatures/config.el

@@ -149,7 +149,7 @@ isn't disabled in `+ligatures-extras-in-modes'."
  ;; leave the check here for safety.
  ((and (> emacs-major-version 27)
        (or (featurep 'ns)
-           (featurep 'harfbuzz))
+           (string-match-p "HARFBUZZ" system-configuration-features))
        (featurep 'composite))   ; Emacs loads `composite' at startup
 
   (after! ligature

modules/ui/ligatures/packages.el

@@ -2,6 +2,6 @@
 ;;; ui/ligatures/packages.el
 
 (when (and (or (featurep 'ns)
-               (featurep 'harfbuzz))
+               (string-match-p "HARFBUZZ" system-configuration-features))
            (featurep 'composite))
   (package! ligature :pin "6ac1634612dbd42f7eb81ecaf022bd239aabb954"))