Fix prefix discovery for files with read-symbol-shorthands (bug#67325)

Fix prefix discovery for files with read-symbol-shorthands (bug#67325)

In a previous commit, the local-variable read-symbol-shorthands is
already read into the temporary buffer used for the autoload parsing
aerobatics, so all we needed to do in 'l-g--compute-prefixes' is
use 'read' to give 'read-symbol-shorthands' a chance to kick in.

* lisp/emacs-lisp/loaddefs-gen.el
(loaddefs-generate--compute-prefixes):

(cherry picked from commit 817140a852e79c5ef3cf7dc5e4c50aa710e8c4a2)