Improve declarations. Use featurep for detection.

- Add more declarations.
- Mark external packages with "ext" in `declare-function`.
- Use `featurep` to detect whether requirements are installed.

company-prescient.el

@@ -66,7 +66,7 @@ This is for use on `company-completion-finished-hook'.")
   "Minor mode to use prescient.el in Company completions."
   :global t
   :group 'prescient
-  (if (not (boundp company-transformers))
+  (if (not (featurep 'company))
       (progn
         (setq company-prescient-mode nil)
         (user-error "`company-prescient-mode': Company not found"))

vertico-prescient.el

@@ -144,7 +144,7 @@ This mode will:
 - advise `vertico-insert' to remember candidates"
   :global t
   :group 'prescient
-  (if (not (boundp 'vertico-sort-function))
+  (if (not (featurep 'vertico))
       (progn
         (setq vertico-prescient-mode nil)
         (user-error "`vertico-prescient-mode': Vertico not found"))