Stop using a dynamically bound 'generated-autoload-file' variable

Stop using a dynamically bound 'generated-autoload-file' variable

* doc/lispref/loading.texi (Autoload): Document change of name
(bug#39823).

* lisp/emacs-lisp/autoload.el (autoload-find-generated-file): Pass
the file name in.
(autoload-generated-file): Ditto.
(autoload-file-load-name): Ditto.
(generate-file-autoloads): Ditto.
(autoload--setup-output): Ditto.
(autoload-generate-file-autoloads): Ditto, and alter doc string to
reflect when `generated-autoload-file' is heeded.
(update-file-autoloads): Pass outfile in to functions.
(autoload-find-destination): Ditto.
(update-directory-autoloads): Make into an obsolete shim around
`make-directory-autoloads'.
(make-directory-autoloads): Renamed from
`update-directory-autoloads' with new semantics.
(batch-update-autoloads): Adjust caller.

* lisp/emacs-lisp/package.el (package-generate-autoloads): Adjust
caller.