plist-get-value treats arg as backquoted

This allows use of variables or even arbitrary expressions to construct
use-package arguments:

(use-package some-package
  :mode ,mode-spec
  :bind (,binding
         ,@more-bindings
         ,@(cl-loop for i from ?a to ?z
                    collect `(,(string i) . nifty-function))))
use plist-get-value for all non-sexp args